Preparation: Heat 2 tablespoons oil in the cooker. Over high heat, brown the oxtails on all sides, in 1 to 3 batches, adding extra oil as needed. Transfer the browned oxtails to a platter and set aside. Saute the onions and garlic in the fat remaining in the pot, stirring frequently, until the onions are browned, about 4 to 5 minutes. Return the browned oxtails to the cooker and stir in celery, stout beer , beef stock, tomato paste, thyme , dry mustard , bay leaf, and parsley. Make certain that there are no bits of onion sticking to the bottom of the cooker. Lock the lid in place and over high heat to bring to high pressure. Adjust heat to maintain high pressure and cook for 55 minutes. Reduce the pressure naturally or use a quick-release method (which will not toughen the oxtails.) Remove the lid, tilting it away from you to allow any excess steam to escape. At this point, the meat should be very tender, falling easily away from the bone when poked with a fork. If not, lock the lid back in place and return to high pressure for an additional 5 minutes. Cool, then refrigerate overnight, if desired. Remove the congealed fat before continuing. About 20 minutes before you plan to serve, remove the bay leaf and parsley. Stir in the turnips, carrots, and small onions. If the gravy is extremely thick, thin it with 1/2 cup water. Lock the lid in place and over high heat bring to high pressure. Adjust the heat to maintain high pressure and cook for 5 minutes. Release pressure with a quick-release method. Remove the lid, tilting it away from you to allow any excess steam to escape. Add salt to taste before serving. Serve the stew with mashed potatoes , rice , or barley to absorb the plentiful gravy. If your schedule permits, cook the oxtails a day or two before, cool and refrigerate, then remove the congealed fat from the top. Yield: 4 servings Recipe Source: Cooking Under Pressure by Lorna J. Sass (William Morrow) Reprinted with permission.