Heat olive oil on medium heat, and saute the onions and garlic for about 2 minutes, stirring frequently. You don't want the garlic to brown. Toss in the mushrooms and stir another 1 minute. Then add the tomatoes and basil and simmer for 15 minutes or so, letting the flavors meld. Stir occasionally to keep from sticking to the bottom. Serve with pasta, spaghetti squash, or with anything else you want a nice sauce with.