Start your grill.

In a saute pan over medium heat, cook onions in 1 tsp olive oil until softened, add cherry tomatoes and cook until softened. Set aside.

Drizzle 1 tsp balsamic glaze inside each mushroom cap

Place 1 slice provolone cheese inside each mushroom cap

Place ¼ of tomato mixture inside each mushroom cap

Top filling in each mushroom cap with 1 tsp basil pesto

Brush all sides of each mushroom with 1 tsp olive oil

Grill until browned and softened, top each mushroom with 1 slice provolone and grill until melted

While mushrooms are grilling, make tomato topping by sauteing thick tomato slices in 1 tsp olive oil over medium heat until browned

Spread each bun with 1 tsp mayo and 1 tsp basil pesto (dividing amount between top/bottom bun)

Place portabella on bun, top with ¼ cup herb salad and 1 cooked thick tomato, drizzle with 1 tsp balsamic glaze and top with bun