Pierce the roast all over with the tip of sharp paring knife to a depth of about ½ inch to allow the seasonings to penetrate the meat.

Crush the garlic in a garlic press into a small mixing bowl. Add the salt, oregano, cumin, and pepper and mash together with fork to make a paste. Place the roast in a nonreactive baking dish. Rub the roast well with the paste and add the bitter orange juice and ½ cup sherry. Sprinkle the onion rings over the top. Cover with plastic wrap and marinate, refrigerated, for at least 4 hours or preferably overnight. Turn the roast once or twice while marinating.

When ready to roast, preheat the oven to 400°F. Remove the roast from the marinade and pat dry; reserve the marinade and onion. Place it in a Dutch oven or roasting pan just large enough to hold it comfortably. Roast for 20 minutes, then lower the oven temperature to 325°F. and add the marinade and onion. Continue to cook, basting every 15 minutes with the pan juices, until the pork reaches an internal temperature of 175°F., 40 to 50 minutes. If the pan juices begin to dry up, add up to ½ cup additional sherry and a little water, if needed. Remove the roast from the oven and let it rest, loosely covered with foil, for 10 to 15 minutes.

Meanwhile, add 1 cup water to the roasting pan and stir to loosen any browned bits. Bring to a boil on top of the stove and cook, stirring, until the juices have reduced to about ½ cup. Slice the pork thinly and serve it with the onion and pan sauce. For a richer pan sauce, swirl the butter into the juices just before serving.