Prepare the slow cooker: Add the chopped onion, browned ground beef, carrots, celery, kidney beans, white beans, diced tomatoes with juice, tomato-basil marinara sauce, oregano, beef broth, salt, and black pepper to the slow cooker. Stir to combine all the ingredients.
Cook the soup: Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours, until the vegetables are tender and the flavors have melded together.
Cook the pasta separately: About 20 minutes before serving, cook the pasta according to the package directions in a separate pot.
Add the finishing touches: Stir the hot pepper sauce into the soup. If desired, mix in the cooked pasta or serve the pasta on the side so it doesn't absorb too much broth while sitting in the soup. Taste and adjust seasoning with additional salt, pepper, or hot sauce if needed.
Serve and garnish: Ladle the soup into bowls and top with grated Parmesan cheese and chopped fresh parsley, if desired.