Pat the oxtails dry and season generously with salt, black pepper, garlic powder, onion powder, paprika, and thyme
Optional: dredge lightly in flour for a thicker gravy
Heat oil in a skillet over medium-high heat
Brown the oxtails on all sides
Transfer the browned oxtails to the crock pot
Add sliced onions, minced garlic, and optional carrots or celery
Pour in beef broth or water until the oxtails are partially covered
Add Worcestershire sauce, bay leaves, and optional tomato paste or browning sauce
Cover and cook on low for 8 to 10 hours or on high for 4 to 6 hours
Check for tenderness; the meat should be falling off the bone
Remove bay leaves before serving
Skim excess fat from the top if needed
Serve with the gravy over rice, mashed potatoes, or noodles