In a 3 quart casserole dish, combine cornstarch, water, oil, onion and lettuce. In a small bowl, mix soy sauce, brown sugar and garlic. Whisk together tomato and soup mixture. Pour this over cornstarch mixture and mix just enough of the potatoes to coat.
Layer 1 layer of potatoes, onions, lettuce, beans, ricotta and mushrooms. Stir in the remainder of the ingredients except tuna. Steam the mixture until tender but hot and able to remove excess moisture, about 10 minutes.
Layer 4 layers of potatoes, onions, lettuce, beans, ricotta, mushrooms and sour cream in a single layer. Sprinkle tomato mixture over all and serve immediately.