In large saucepan bring about 3 quarts water to boil. Add pasta. Return to boiling, cook for 9 minutes. Add frozen mixed vegetables. REturn to boiling, cook for 5-7 minutes more til pasta is tender but slightly firm and vegetables are crisp tender. Drain, rinse with cold water and drain again.
For sauce, in a large saucepan cook onion in margarine or butter till tender. Stir in flour and bouillon granules. Add milk all at once. Cook and stir till thickened and bubbly. Add cheese, stirring till 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 qt baking dish. Cover with foil.
Bake in a 375 oven for 20 minutes. Remove foil. Arrange tomato slice halves on top of pasta mixture. Bake uncovered for 5-10 minutes more until heated through. Let stand 10 minutes before serving.