Preheat oven to 350°F (175°C).

Dice the celery, onion, and mushrooms.

Sauté the celery, onion, and mushrooms in a pan until just tender.

If using frozen vegetables, thaw them completely. If using fresh vegetables, cook them until tender, then drain any excess water.

In a large bowl, mix together the cream of chicken soup, sour cream, black pepper, garlic powder, and onion powder.

Stir in the vegetables, sautéed celery, onion, and mushrooms, 1 1/2 cups of shredded Swiss cheese, and 1/2 of the French fried onions.

Spoon the vegetable mixture into a 3-quart shallow baking dish.

Cover the baking dish and bake for 30 minutes.

Remove the cover and stir the mixture.

Sprinkle the remaining Swiss cheese and French fried onions evenly over the top of the casserole.

Bake uncovered for an additional 5-10 minutes, or until the cheese is melted and bubbly and the onions are golden brown.

Let the casserole stand for 5 minutes before serving.