California Chicken Casserole

INGREDIENTS
16-ounce bag frozen California blend vegetables, thawed*
2 cups chopped, cooked chicken
4 ounces canned mushrooms, drained
2 stalks celery, finely chopped
1 small onion, chopped
2 spoonful butter
1/4 cup mayonnaise
1/4 teaspoon salt or to taste
1/2 teaspoon pepper
1/8 teaspoon garlic powder
2 ounces bell peppers, chopped
6 ounces Swiss cheese, grated