Preheat oven to 350°F (175°C).

Thaw and drain the mixed vegetables.

In a large bowl, combine the thawed vegetables, cream of mushroom soup, half of the shredded Swiss cheese, sour cream, ground black pepper, and pimento (if using).

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

Pour the mixture into a 1-quart casserole dish.

Cover the casserole dish with a lid or aluminum foil.

Bake in the preheated oven for 30 minutes.

Remove the casserole from the oven and uncover it.

Sprinkle the remaining Swiss cheese and onion rings on top.

Return the casserole to the oven and bake, uncovered, for another 5 minutes, or until the cheese is melted and bubbly.