Line a rimmed baking sheet with parchment paper. With a pastry brush, brush olive oil on outside of each mushroom cap, and around the interior and edges.
Place oiled caps on lined baking sheet and evenly divide the mozzarella cheese between them. Top each cap with sliced grape tomatoes and drizzle with pesto. Sprinkle with salt and pepper, according to taste.
Place baking sheet on middle rack of oven and bake for approximately 15 minutes, or until cheese is melted. Remove from heat and drizzle with aged balsamic vinegar or a thicker glaze, if desired. Serve immediately.