Preheat oven to 350°F (175°C).

Thaw and drain the frozen vegetable combination (broccoli, carrots, and cauliflower).

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

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

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

Bake the casserole, covered, for 30 minutes, or until the vegetables are tender.

Remove the casserole from the oven and uncover.

Sprinkle the remaining Swiss cheese and onion rings in diagonal rows across the top of the casserole.

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

Let stand for a few minutes before serving.