In large fry pan heat oil and add ox tail. Cook until golden brown and edges start to crisp. Add the remaining portion of the onion sliced thinly, not used in salsa.

On a comal or flat griddle char all of the chiles, 1/4 of the onion, the garlic and the tomatoes. Put all these ingredients into a blender with salt to taste and the water. Blend until smooth.

Add salsa to fry pan with ox tail and let simmer for about 30 or until fork tender.