Over medium heat, cook the pork sausage and ground beef in a large skillet or pot until both are browned and cooked through. Drain off any fat from the pan.
Add the chopped onion and cook for 3-4 minutes, occasionally stirring, until softened.
Stir in diced tomatoes, tomato paste, cider vinegar, chili powder, garlic powder and red pepper flakes (if using). Simmer for 10 minutes to allow flavors to blend together.
Add the shredded cabbage and stir it into the mixture to combine everything together well. Cover with a lid and simmer on low heat for 25-30 minutes, occasionally stirring until cabbage is tender but still firm.
Taste and season with salt and pepper as desired. Serve hot.