• 6 large portobello mushroom caps, wiped clean with a damp cloth and stems carefully removed (reserve stems for other uses like stock or stir-fries)
• 1/4 cup balsamic glaze (reduction), either a good quality store-bought version or fresh homemade (see instructions below)
• 2 tablespoons vegetable oil, or a light olive oil, for sautéing and grilling
• 2 large sweet onions, such as Vidalia or Walla Walla, peeled and thinly sliced into even rounds
• Salt and freshly ground black pepper, to taste for seasoning the onions and mushrooms
• Pinch of sugar, optional, to enhance the caramelization of the onions
• 6 ounces crumbled blue cheese, such as Gorgonzola, Roquefort, or Stilton, for its distinctive tangy flavor
• 6 soft sandwich rolls, brioche, ciabatta, or your favorite burger buns for serving

1. **Prepare the Portobello Mushrooms:** Begin by gently cleaning the portobello mushroom caps. Use a dry cloth or a soft mushroom brush to wipe away any dirt or debris. Avoid rinsing them under water, as mushrooms tend to absorb moisture, which can make them soggy. Carefully twist or cut off the tough stems and set them aside; they are excellent for adding flavor to vegetable stocks or other culinary uses.
2. **First Glaze Application:** Measure out your balsamic glaze. Using a pastry brush, generously coat the smooth, rounded tops of all six cleaned portobello mushroom caps with approximately half of the balsamic glaze. Place the glazed mushrooms on a plate or tray and allow them to marinate slightly while you proceed with preparing the onions. This initial coat will infuse the mushrooms with flavor.
3. **Begin Sautéing the Onions:** Peel and thinly slice the two large sweet onions into uniform rounds or half-moons. In a large, sturdy skillet or sauté pan, heat the vegetable oil over medium heat until it shimmers. Add the sliced onions to the hot pan, spreading them evenly. Season generously with a pinch of salt and freshly ground black pepper to begin building their flavor.
4. **Caramelize to Perfection:** Continue to sauté the onions, stirring frequently, until they become beautifully soft, translucent, and begin to achieve a rich golden-brown caramelization. This process typically takes between 5 to 7 minutes. For an extra touch of sweetness and to aid in the browning, you can add a small pinch of sugar during this stage. Once caramelized, remove the onions from the skillet and set them aside.
5. **Preheat the Grill:** While the onions are cooking, preheat your gas or charcoal grill to a medium temperature. Aim for an internal temperature of approximately 400°F (200°C). Proper preheating is essential for achieving good grill marks and even cooking.
6. **Initial Grilling of Mushrooms:** Carefully oil the hot grill grates to prevent the mushrooms from sticking. Place the balsamic-glazed mushroom caps onto the prepared grates, with the gill-side facing upwards. Grill for exactly 3 minutes; this allows the top side to cook gently and the glaze to adhere.
7. **Flip, Glaze, and Finish Grilling:** Using sturdy tongs, carefully flip each mushroom cap over. Now, liberally brush the remaining balsamic reduction onto the gill-side of each mushroom. Continue grilling for approximately 3 minutes more, or until the mushrooms are tender when pierced with a fork and slightly softened. Cooking times may vary slightly based on the thickness of your mushrooms and the heat of your grill.
8. **Melt the Blue Cheese:** Once the mushrooms have reached desired tenderness, sprinkle an even portion of the crumbled blue cheese onto the top (gill-side) of each hot mushroom cap. If your grill has a lid, close it, or cover the mushrooms loosely with aluminum foil. Cook for just about 1 minute, or until the blue cheese has softened and started to melt into a creamy, luscious topping. Be cautious not to overcook the cheese, as it can dry out quickly.
9. **Assemble and Serve Immediately:** To assemble your magnificent burgers, place a generous helping of the warm, sautéed caramelized onions onto the bottom half of each soft sandwich roll. Carefully transfer a cheese-topped portobello mushroom onto the onions. Complete the burger with the top bun and serve immediately to enjoy the warmth and exquisite blend of flavors. For added crunch and freshness, consider adding a crisp lettuce leaf, a slice of ripe tomato, or some tangy pickles.

How To Make A Homemade Balsamic Reduction (Glaze)

Elevate your cooking with a simple, homemade balsamic reduction. It’s surprisingly easy and adds a gourmet touch to many dishes:

1. **Preparation:** Pour 1 cup of good quality balsamic vinegar into a small, non-reactive saucepan (stainless steel or enameled cast iron are ideal). Avoid aluminum, as it can react with the vinegar.
2. **Slow Simmer and Reduce:** Bring the vinegar to a gentle simmer over medium heat. It’s crucial not to let it boil vigorously; a slow, steady simmer is key for the best flavor and consistency. Continue cooking, stirring occasionally to prevent it from sticking to the bottom of the pan, until the vinegar has thickened to a syrupy consistency that easily coats the back of a spoon. This process typically takes between 12 to 20 minutes, depending on the heat level and the width of your saucepan. The wider the pan, the faster it will reduce.
3. **Cool and Store:** Once it reaches the desired consistency, remove the saucepan from the heat. Remember that the balsamic glaze will continue to thicken significantly as it cools. Allow it to cool completely in the saucepan before transferring it to a small, airtight jar or bottle. Store it covered in the refrigerator, where it will remain fresh and flavorful for several weeks, ready to enhance your next meal.

**Yields:** Approximately 1/2 cup of rich, homemade balsamic glaze.

Pro Tips for Crafting the Perfect Portobello Burger:

• **Selecting Portobellos:** When choosing portobello mushrooms, look for caps that are large, firm, and relatively flat. This shape will make them easier to grill evenly and fit perfectly onto your burger buns.
• **Cleaning is Key:** Always use a damp cloth or a dedicated mushroom brush to clean portobellos. Washing them under running water can cause them to absorb excess moisture, leading to a watery burger.
• **Blue Cheese Alternatives:** If the bold flavor of blue cheese isn’t for you, there are many delicious alternatives. Consider using slices of smoked gouda, provolone, fresh mozzarella, or a sharp cheddar for a different yet equally delightful cheesy topping.
• **The Right Bun:** While soft sandwich rolls are a classic choice, don’t hesitate to experiment with different types of buns. Ciabatta rolls offer a nice chew, while brioche buns add a touch of richness and sweetness. Lightly toasting your chosen buns on the grill before assembly will add an extra layer of flavor and texture.
• **Adding a Kick:** For those who enjoy a bit of heat, sprinkle a pinch of red pepper flakes into the onions while they sauté, or add a dash of your favorite hot sauce to the balsamic glaze for an extra zing.
• **Complete Your Meal:** To make these burgers a complete and wholesome meal, serve them alongside crispy sweet potato fries, a refreshing green salad with a light vinaigrette, or some grilled corn on the cob.

Nutrition Facts (Per Serving)