Preheat oven to 375°F (190°C).

Cook frozen cauliflower and broccoli according to package directions.

Drain the cooked cauliflower and broccoli thoroughly.

Cut up any large pieces of cauliflower and broccoli into smaller, bite-sized portions.

In a large bowl, combine the creamed corn and whole kernel corn.

Add the shredded Swiss cheese and condensed cream of celery soup to the corn mixture.

Stir the cheese and soup into the corn until well combined.

Gently fold in the cooked cauliflower, broccoli, and sliced mushrooms into the corn mixture.

Transfer the vegetable mixture to a 13 x 9 x 2-inch baking dish.

Melt the margarine.

Toss the onion rings with the melted margarine to coat them evenly.

Sprinkle the margarine-coated onion rings evenly over the top of the vegetable mixture in the baking dish.

Bake the vegetable bake, uncovered, in the preheated oven for 30 to 35 minutes, or until golden brown and bubbly.

Let cool slightly before serving.