Roast 2 whole red peppers in a 450 F degree oven for 25-30 minutes. Remove from oven and place in a bowl and cover with a plate or plastic wrap until cool. Remove skin of bell pepper.
Brush the portobello mushroom on one side with olive oil and season with salt and pepper. Grill for 3-5 minutes until browned. Repeat on the reverse side.
Place portobello mushroom onto your choice of bread. Top mushroom burger with roasted red pepper, fontina cheese, and pesto.