- 1/2 portobello mushroom
- 1/4 cup plus 1 teaspoon prepared balsamic vinaigrette, divided
- 1 teaspoon olive oil
- 1 teaspoon butter
- 1/2 cup sliced onions
- 1/4 cup sliced roasted red peppers
- 1/4 baguette, sliced lengthwise
- 2 slices extra-sharp cheddar cheese
Clean portobello mushroom and remove stem. Marinate cap in 1/4 cup balsamic vinaigrette for at least 30 minutes.
Preheat a grill or grill pan to medium heat and lightly oil the grate. Grill mushroom cap until tender, 15 to 20 minutes, turning occasionally. Let cool slightly, then thinly slice.
Meanwhile, heat oil and butter in a heavy skillet over medium heat. Add onions and cook, stirring frequently, until deep golden, 15 to 20 minutes. Stir in sliced mushrooms, peppers and remaining 1 teaspoon balsamic vinaigrette.
Lightly toast baguette under the broiler. Top one half with vegetable mixture and cheese. Return to the broiler and cook until cheese is melted, 1 to 2 minutes. Top with remaining bread half and serve immediately.