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