PREHEAT oven to 375° F.
MELT butter in medium saucepan. Add onion; cook for 5 minutes. Blend in flour, salt and ground black pepper. Stir in evaporated milk. Cook over medium heat, stirring constantly, until mixture comes just to a boil. Stir in cheddar cheese and Parmesan cheese until melted.
COMBINE rice, parsley, paprika, oregano, cayenne pepper, and tomatoes in 11 x 7-inch baking dish. Top with tuna. Spread cheese sauce over tuna.
BAKE for 20 to 25 minutes or until edges are bubbly.