Great for a light supper, paired with a salad and a dish of Mercers ice cream for dessert.
Heat oil in large skillet over medium heat. Add onion and slowly sauté for 20 minutes until browned and caramelized. Add garlic and cook for 30 seconds. Add the tomatoes, mushroom stems, oregano and wine. Season lightly with salt and pepper.
Simmer the sauce for 20 minutes, then stir in the basil. Remove from heat, cool slightly, and place in blender or food processor. Puree until smooth. Preheat the oven to 475. Spray foil lined baking sheet with nonstick cooking spray.
Place mushroom caps on the prepared pan. Spoon 3 tablespoons sauce onto each cap. Top each with 2 slices of cheese and top with remaining sauce. Sprinkle with grated Parmesan and olives, if using. Bake for 10 to 12 minutes or until cheese is bubbly. Cool slightly and serve warm.