Place the sauerkraut and onion in a large skillet over medium-low heat with enough water to cover. Simmer 1 hour, or until most of the water has cooked off.

Bring a large pot of lightly salted water to a boil. Add rotini and cook for 8 to 10 minutes or until al dente; drain.

In a medium skillet over medium heat, saute the mushrooms in 2 tablespoons of the butter for about 5 minutes.

Stir the remaining butter, cooked pasta, and cream of mushroom soup into the sauerkraut mixture. Cook and stir 15 minutes, or until heated through.