Purée the onion and garlic in a food processor with a dash of olive oil and a good pinch of salt. Tip into a bowl with the oxtail and toss to coat. Cover and refrigerate overnight.
Heat a little olive oil in a large, heavy-based casserole over a medium-high heat. Drain the oxtail, reserving the marinade, and wipe off any excess. Fry the oxtail in batches until golden brown all over then transfer to a plate with a slotted spoon. Deglaze the pan with the wine then add the stock, tinned tomatoes, bay leaf, Worcestershire sauce and reserved marinade. Bring to the boil, return the oxtail to the pan then reduce the heat to a simmer. Cover with a lid and cook gently for 3 hours until tender, topping up with a little more stock during cooking if necessary.
Carefully remove the oxtail from the pan with a slotted spoon then increase the heat and reduce the sauce for a few minutes until thickened. Add the watercress then return the oxtail to the pan. Reheat gently then season to taste.
Meanwhile make the polenta. Bring the stock to the boil in a saucepan then quickly whisk in the polenta (make sure you whisk continuously to prevent the polenta from turning lumpy). Turn the heat down and keep whisking for 2 minutes or until the polenta has thickened and starts to pull away from the sides of the pan. Remove from the heat, stir through the butter, cream and morcilla and season to taste.
Serve the oxtail with the polenta and garnish with chopped chives and tomato concasse.