In a large saucepan or Dutch oven bring about 3 quarts water to boiling. Add pasta. Return to boiling; cook for 9 minutes. Add frozen mixed vegetables. Return to boiling; cook for 5 to 7 minutes more or until pasta is tender but slightly firm and vegetables are crisp-tender. Drain; rinse with cold water. Drain again.
For sauce, in a large saucepan cook onion in margarine or butter until tender. Stir in flour and bouillon granules. Add milk all at once. Cook and stir until thickened and bubbly. Add cheese, stirring until melted. Stir in cilantro or parsley. Remove from heat.
Add pasta and chicken or tuna to the sauce; toss to coat. Spoon mixture into a 2-quart square baking dish. Cover dish with foil.
Bake in a 375 degree F oven for 20 minutes. Remove foil from dish. Arrange tomato slices on top of pasta mixture. Bake, uncovered, for 5 to 10 minutes more or until heated through. Let stand 10 minutes before serving. Makes 6 servings.