Cook pasta according to package directions, adding green beans and broccoli the last 2 minutes; drain. Rinse with cold water; drain again. Return pasta to pan.
Meanwhile, in a large skillet heat oil over medium heat. Add mushrooms, red peppers and chicken. Sprinkle with Italian seasoning, 1/8 teaspoon salt, and 1/8 teaspoon pepper. Cook and stir until chicken is no longer pink and vegetables are tender. Add chicken to pasta in pan.
For cheese sauce, in a large saucepan melt the 1/4 cup butter over medium heat. Add onion and garlic; cook and stir until tender. Stir in flour, then tomato paste. Gradually stir in milk. Cook and stir until thickened and bubbly; reduce heat. Gradually stir in cheddar and American cheeses until nearly melted. Remove from heat; season to taste with additional salt and pepper. Add cheese sauce to pasta mixture, stirring to coat.
Spoon pasta mixture into a 3 quart baking dish. Cover baking dish with plastic wrap, then with foil. Freeze for up to 4 months.
To serve, thaw in the refrigerator overnight. Preheat oven to 350 degrees F. Remove plastic wrap; recover with foil. Bake about 1 hour 15 minutes, or until heated through, stirring every 30 minutes. Top with crushed croutons.