Heat oven to 425 degrees. Arrange tomatoes on a sheet pan, with half the garlic, and drizzle with 2 tablespoons of olive oil. Season with 1/4 teaspoon of salt and 1/4 teaspoon of black pepper. Toss to coat the tomatoes.
Add the mushroom caps in between the tomatoes, gill side up, and drizzle each generously with olive oil, another 2 tablespoons or more. (Don’t skimp here, as the olive oil will add lots of rich flavor.) Scatter the mushrooms with the remaining garlic, and season each mushroom with 1/4 teaspoon salt and 1/4 teaspoon black pepper. Fill each mushroom with marinara sauce, and top with cheese. If sauce and cheese spill over, that’s totally fine. Place sheet pan in the oven and bake for 20 minutes or until cheese is golden brown and bubbly.
Remove the pan from the oven. Turn on the broiler to high temperature.
Meanwhile in a small bowl, combine gluten free panko crumbs, basil, 1 tablespoon of olive oil, grated Parmesan (if using), and a grind or two of fresh pepper. Then place a generous helping of the panko topping on each mushroom cap. Put sheet pan back into the oven for one minute. Keep a close eye on the mushrooms to avoid burning the panko.
Garnish mushrooms with basil leaves. Either serve directly from the sheet pan or transfer mushrooms to serving plates, along with a few of the roasted tomatoes.