Peel and slice the onion and fry it gently in the butter or oil in a saucepan, until soft (2 to 3 minutes).
Wash and slice the mushrooms and pepper. Add them to the onion and fry gently, until soft (2 minutes). Add the peas, tomato soup, salt, pepper and garlic Gently stir in the drained tuna and cook for a few minutes, until hot.
Drain and fork the rice. Spoon the rice onto a plate, press into a ring and pour the tuna sauce into the middle.