Portabella Pesto Sandwich with Balsamic Red Onions

I am especially proud of this vegetarian sandwich because my meat loving husband absolutely loves it. Portobellas have a natural meatiness, so it doesn't feel like you're skimping on heartiness here. The roasted peppers and balsamic red onions are complemented by a delicious pesto mayonnaise and melted provolone cheese. Put together on a grilled roll (or baguette), this sandwich will quickly become one of your favorites too!
Serves 2

Ingredients:
2 tbsp. pesto (recipe to follow)
2 tbsp. mayonnaise
3 slices provolone
1 small red onion, sliced
2 portabellas
1/3 cup olive oil
1 sweet Italian red pepper (or a bell pepper)
1 tsp. brown sugar
2 tbsp. balsamic vinegar
salt, black pepper
2 deli rolls (or baguette pieces)

Assembly:
Turn portabellas in olive oil to coat. Do the same for the pepper. Grill the pepper over high heat for 10 or so minutes until the skin is black (an indoor grill, grill pan or oven all work). Remove to a paper bag for five minutes to allow the skin to fall off. Reduce heat to medium high and grill the portabellas about 5 minutes on the first side, about 3 minutes on the second (until you have nice grill marks and the mushroom appears cooked). While the portabellas are grilling, saute the onions in 1 tbsp. olive oil (you can do this right on a grill pan if you're cooking indoors). Lower heat to medium-low and cook about 10 minutes before adding vinegar, brown sugar, salt and pepper. Cook for another 2-3 minutes until the vinegar is absorbed. You will have removed the portabellas by now. Let them sit for a few minutes before cutting them into 1/2 strips. Remove the skin and insides (membranes and seeds) from the red pepper and slice. Mix the pesto and mayonnaise. Place each side of the rolls on a baking sheet (your oven should be preheated to 400). Assemble your sandwich (mushroom, red pepper, red onion mixture) then top with provalone. Bake for about 5 minutes until cheese begins to melt and the edges of the bread appear crispy. Remove from oven, spread pesto mayonnaise on the top side of the roll and press the sanwich together. Serve immediately.