Preheat oven to 350 degrees F (175 degrees C).

Thaw and drain the frozen broccoli, carrots, and cauliflower.

In a large bowl, combine the thawed vegetables, cream of mushroom soup, 1/2 cup shredded Swiss cheese, sour cream, black pepper, and drained pimento (if using).

Add half of the can of French fried onion rings to the mixture and stir well.

Pour the vegetable mixture into a 1-quart casserole dish.

Cover the casserole dish with a lid or aluminum foil and bake for 30 minutes.

Remove the casserole dish from the oven and carefully remove the lid or foil.

Top the casserole with the remaining 1/2 cup shredded Swiss cheese and the remaining French fried onion rings.

Return the casserole dish to the oven and bake uncovered for an additional 5 minutes, or until the cheese is melted and bubbly and the onion rings are golden brown.

Let the casserole cool slightly before serving.