Heat oil in a 10- to 12-inch frying pan over medium heat. When hot, add onions and garlic. Stir frequently until onions are limp and beginning to brown, about 6 minutes. Pour into a 2- to 3-quart baking dish with sides at least 2 inches high, and spread onions level. Top evenly with tomato slices, basil, and oregano. Sprinkle generously with salt and pepper.
In a food processor, whirl bread with butter and cheese until mixture forms coarse crumbs. Sprinkle evenly over tomatoes.
Bake in a 350° oven until topping is golden brown and juices are bubbling, 20 to 25 minutes. Cool 5 minutes and serve warm.