To make this recipe we are going to use a cast iron saucepan or cocotte since we are interested in the high temperature that it achieves inside as well as the sustained boiling that is obtained by covering it and putting the fire to a minimum during cooking. Failing that, you can use an express pot but without using pressure.
We begin by browning the surface of the shank, so that it has a nice color on the outside and to begin to add flavor to the oil in which we will later cook the onions. While the morcillo is browning, finely chop the onions. Once the meat is ready, we remove it and reserve.
In the oil used to brown the shank and a little more that will have to be added, we fry the onions, adding a little salt and lowering the heat so they cook slowly without burning. We leave them until their sugars begin to change their color from purple to gold. Then we reintegrate the meat and add the Sherry wine, and after a couple of minutes the meat broth.
We cover the cocotte and let it cook for 3 hours with the fire at a minimum, turning the piece every half hour and preventing it from clinging to the bottom. After that time, the meat will be tender, mellow, and it will seem that it is falling apart, and the sauce will have an intense and unforgettable flavor.