In a large skillet, heat the oil over medium-high heat.
Season the oxtail with salt and pepper to taste. Sear the oxtail in the hot skillet on each side for 2-3 minutes. Transfer beef to the slow cooker.
Add leeks, onion, garlic, red wine, plum tomatoes, tomato paste, stock, Worcestershire sauce, thyme, rosemary, and bay leaves.
Cover and cook on low for 4 -6 hours or high for 2 -3 hours, or until the meat falls away from the bone. Note: you can also cook overnight on low.
Strip the meat from the bones. Discard the bones and remove any excess fat.
Add carrots, celery, and potatoes. Cover and cook on high for 3 hours, or until vegetables are fork-tender. Taste and add more salt and pepper if necessary.
In a small bowl, whisk together flour and 1/2 cup stew broth ultil no lumps. Stir in flour mixture into the slow cooker along with the frozen peas. Cover and cook on high heat for an additional 30 minutes, or until thickened.
Serve immediately, garnished with parsley, if desired.