Place oxtails in a large resealable bag. Mix together remaining oxtail marinade ingredients and pour over oxtails in the bag. Coat the oxtails well and press out air. Marinade overnight.
Remove oxtails from marinade and set the marinade/onion mixture aside. In a braiser or oven safe skillet brown each side of oxtails on medium-high heat in batches. Set oxtails aside. Add onion mixture to pan and sautee. Once sauteed add the oxtail back into pan.
Add water to cover 1/4 of the oxtails. Cover and place in 350 degree oven for 1 hour. Add in tomatoes, cover and return to oven for 30 minutes or until oxtails are tender.
Remove from oven and garnish with drained onion tomato mixture.