Add beef broth, soy sauce, and tomato paste to the skillet, stirring to combine and scraping up any browned bits.
Pour the mushroom gravy mixture over the meatballs in the slow cooker.
Cover and cook on low for 6-8 hours or on high for 3-4 hours until meatballs are cooked through and tender.
If the gravy needs thickening, mix cornstarch or flour with a little cold water and stir into the slow cooker, cooking an additional 15-20 minutes until thickened.
Notes
Add a splash of red wine to the gravy for extra richness.
Replace bread crumbs with almond flour for a lower-carb option.
Add fresh thyme or rosemary to the gravy for an herbal note.
Serve over mashed potatoes, egg noodles, or cauliflower rice.
Mix grated Parmesan cheese into the meat mixture for extra flavor.
Store leftovers in an airtight container in the fridge up to 3 days or freeze for longer storage.
Reheat gently with a splash of beef broth to loosen the sauce if needed.