Pre-heat oven to 400F degrees.
Cut the tomatoes into quarters and place them on a large cookie sheet. Sprinkle with oil, oregano and salt and pepper to taste.
Slice off the end of a garlic head (root side) and place the whole head in a small glass bowl. Sprinkle with oil and place the glass bowl on the cookie sheet with the tomatoes.
Bake for about 45 minutes.
When done, remove from oven. Remove the garlic cloves from the peel using the teeth of a fork. Set aside.
Heat a cast iron or saucepan and add the roasted garlic and tomatoes. Add the mushroom sliced in half and cook for 5 minutes.
Remove from heat. Add fresh basil and parmesan cheese (if desired). Serve hot and store remaining sauce in an air tight jar or glass bowl with a tight lid. Keep refrigerated.