Brush mushroom caps with pesto, using about ¼ cup, and season with salt and pepper.

Brush olive oil onto grill, then heat grill to medium heat.

Add mushrooms to grill and cook 5 minutes with rounded side down.

Flip over, then add mozzarella to on top of mushroom caps.

Close grill and cook another 4-5 minutes, until cheese is melted.

Put buns on grill for about 1 minute to lightly toast.

Spread 1 tablespoon pesto on each bun.

Top with mushroom caps and tomato slices.

Enjoy!