Put mushrooms, smooth side on top, on a baking sheet that has parchment paper. Cook in a preheated oven of 425 F for 5 minutes. Remove from oven and let cool a little bit. Flip the mushrooms, hollow side on top.
In a bowl, mix tomatoes, olives, feta, garlic and basil. With a spoon, distribute this mixture in the mushrooms. Keep cooking at 425F 8 to 10 minutes or until mushrooms are tender and hot.