Preheat oven to 400º F. Spray a small rimmed baking sheet with cooking spray. Set aside.
Remove stems from the mushrooms and place them on the prepared baking sheet. In a small bowl add the olive oil and minced garlic and stir to combine. Brush the mushrooms with the olive oil mixture and bake for 15 minutes, or until browned and bubbly. Remove from the oven and allow to cool slightly.
While the mushrooms are roasting, add the diced tomatoes, pine nuts, minced basil, salt and pepper to a small bowl and stir gently to combine. Spoon into the warm mushroom caps, pressing lightly, and serve immediately.