1. Heat olive oil in a large casserole and saute the onions until lightly browned, 3 to 4 minutes, stirring often.
2. Add garlic, saute for 1 minute, then stir in mushrooms and thyme. Continue to cook over high heat until the mushrooms have softened and given off their juices, 2 to 3 minutes.
3. Add tomatoes and cook until the mixture is thick, about 10 minutes. Serve over wheatberry, bulgur wheat pilaf or soft cornmeal polenta. Ragu can be cooled and refrigerated for up to 4 days, Garnish with additional thyme, if desired.