Cheesy Chicken and Mostaccioli

2 cups loose-pack frozen mixed vegetables

1 medium onion, chopped (1/2 cup)

2 tablespoons margarine or butter

2 tablespoons all-purpose flour

2 teaspoons instant chicken bouillon granules

2 cups milk

1-1/2 cups shredded Monterey Jack cheese with jalapeno peppers (6 ounces)

2 tablespoons snipped fresh cilantro or parsley

1-1/2 cups chopped cooked chicken (about 8 ounces) or 1 9-1/4-ounce can tuna, drained and broken into chunks

1 medium tomato, halved and sliced