Chicken wild rice casserole

2 small onions, chopped
1 cup butter
5 tablespoons corn starch
1 teaspoon chicken bouillon
1/2 teaspoon pepper
2 cans (14-1/2 ounces) chicken broth
2 cups heavy cream
1 bag Tyson pulled cooked chicken
4 cans cooked wild rice
4 sticks celery
1 bag carrots

In a large saucepan, sauté onion, celery and carrots in butter until tender. Stir cornstarch into broth until blended. Gradually stir in broth; bring to a boil. Boil and stir for two minutes or until thickened and bubbly. Stir in the cream, chicken and rice; heat through. Transfer to a greased baking dish. Bake uncovered at 350 degrees for 30-35 minutes or until bubbly.