1 Heat oven to 375 ° F. Remove stems from mushrooms. (Reserve stems to cut up and use in soups, stir-fries, etc). Lightly brush both sides of mushrooms with oil; arrange on a baking pan.2 Bake for 10 minutes, turning once. Meanwhile, in a small bowl combine spinach and tomato sauce. In another bowl, combine green onions, sun-dried tomatoes, and crushed red pepper. 3 Remove mushroom caps from the oven; place gill-side up on baking pan. Spoon spinach mixture into mushrooms, dividing evenly. 4 Top each with an equal amount of the sun-dried tomato mixture and sprinkle with fontina cheese.5 Return to oven until heated through and cheese begins to melt, 5 to 8 minutes. Serve with additional tomato sauce, if desired.