Set your oven to 350 degrees Fahrenheit to warm up.
Prepare the rotini according to the package directions, then drain and keep aside.
Fry the bacon slices in a large skillet until crispy.
Take the bacon out and place it on a plate lined with paper towels to drain.
Drain the bacon fat, leaving about 3 tablespoons in the skillet.
Add the sauerkraut to the skillet with the reserved bacon grease and stir.
Mix in the chopped onion and season with salt and pepper. Cook on medium heat, stirring once in a while, for roughly 10 minutes.
Transfer the cooked noodles into a 9x13 inch baking dish.
Break the bacon into small pieces and toss them with the noodles.
Add the sliced mushrooms and one can of cream of mushroom soup and mix everything thoroughly.
Combine the sauerkraut mixture with the noodles and mushroom soup mixture, mixing well.
Spread the second can of cream of mushroom soup evenly over the top of the dish.
Bake the casserole uncovered for about 45 minutes until heated through and bubbly.
