Marinate (optional but flavor‑boosting): In a bowl, combine oxtails, onions, scallions, garlic, thyme, red pepper flakes, paprika, bay leaves, Worcestershire-style sauce, soy sauce, and ketchup. Stir well, cover, and marinate in the fridge for at least 4 hours or overnight.
Sear the oxtails: Heat a splash of neutral oil in a skillet over medium-high. Brown oxtails on all sides (do not crowd); transfer to slow cooker.
Layer the remaining ingredients: Add the reserved marinade, carrots, chopped tomatoes, tomato paste, and beef broth to the cooker. Ensure oxtails are partially submerged.
Slow‑cook gently: Cook on High for 7–8 hours (or Low for 8–10 hours), uncovering and ensuring liquid still gently covers the meat every hour.
Finish with beans: With about 1 hour left, stir in butter beans. If needed, add more broth or water to maintain coverage. Remove any excess fat from the surface.
Final seasoning: In the last half‑hour, adjust seasoning—add salt and pepper to taste, skim fat if needed.
Rest before serving: Let the stew rest 10 minutes before serving. Serve over rice, polenta, or with crusty bread to soak up the rich sauce.