Place a grill pan over medium-high heat or preheat a gas or charcoal grill.
Drizzle 3 tablespoons of olive oil over both sides of the mushrooms, rubbing it in good. Sprinkle the mushrooms with salt and pepper. Brush olive oil on the grill to prevent the mushrooms from sticking. Grill until the mushrooms are heated through and tender, about 5 minutes each side.
Preheat the oven to 400*F.
Spread 1/2 cup of the marinara sauce on the bottom of a 9"x13" baking dish. Place the grilled mushrooms on top of the marinara sauce and top with the remaining marinara sauce. Sprinkle with the mozzarella and Parmesan cheeses and top with the butter pieces. Bake until the cheese melts and the top is golden, about 15 minutes. Serve and enjoy!