One 2-pound pork top sirloin roast, trimmed of all visible fat
3 tablespoons ketchup
2 tablespoons sherry vinegar
1 1/2 teaspoons soy sauce
1 tablespoon coarsely chopped peeled fresh ginger
1 tablespoon chopped garlic
1/2 teaspoon five-spice powder
Melt the butter in an enameled cast-iron casserole in which the roast will fit snugly. Season the meat with salt, add it to the casserole and cook over moderate heat, turning as necessary, until well browned all over, about 15 minutes. Pour off the fat from the pan.
Meanwhile, in a food processor, combine all of the remaining ingredients with 1/4 cup of water and puree until smooth.
Pour the sauce over the meat and bring to a boil. Reduce the heat to very low, cover and cook for 1 hour, turning the meat every 15 minutes; it is done when an instant-read thermometer inserted in the thickest part registers 160°. Remove from the heat and let the meat rest in the sauce for 10 minutes; this helps tenderize it and makes it easier to slice.
Carve the roast into 1/4-inch slices. Stir any carving juices into the sauce and reheat if necessary. Season the sauce with salt and spoon it over the meat.
The ginger and soy notes in the sauce give the pork an Asian accent that finds its best red wine match in a fruity Pinot Noir. Or try a spicy, rich dry white, such as an Asatian Gewürztraminer.