Set your oven to heat at 350 degrees Fahrenheit.
Prepare the rotini according to the package directions, then drain and keep aside.
Fry the bacon in a large skillet until crisp.
Transfer the cooked bacon to a paper towel-lined plate to drain the grease.
Leave behind 3 tablespoons of the bacon fat in the skillet.
Add the sauerkraut to the skillet with the reserved bacon grease and stir well.
Mix in the chopped onion, season with salt and pepper, and cook on medium heat for roughly 10 minutes, stirring occasionally.
Put the cooked rotini into a 9x13 inch baking pan.
Crumble the bacon and mix it evenly with the noodles.
Incorporate the drained mushrooms and one can of cream of mushroom soup, stirring thoroughly.
Add the sauerkraut and onion mixture and toss everything together.
Evenly spread the remaining can of cream of mushroom soup over the entire dish.
Bake the casserole uncovered for about 45 minutes until heated through and bubbly.