Note: When Kathie Jenkins started exploring cooking under pressure, one of the real winners was this Mexican-flavored stew. After an hour in the pressure cooker, the meat becomes flavorful and so tender that it literally falls apart. The tasters in The Times Test Kitchen couldn't get enough. Don't be concerned if the roast has to be wedged into the pot; it will shrink considerably during cooking. And above all, don't even think about the fact that it came from a cookbook published by Presto, the Eau Claire, Wis., company that manufactures pressure cookers.
Place cooking rack in 6-quart pressure cooker. Trim as much surface fat from meat as possible. Sprinkle entire roast with salt and pepper to taste. Place meat and sliced onion on rack. Add boiling water. Lock lid in place, bring up to full pressure and cook 60 minutes. Allow pressure to come down naturally. Remove meat and rack.
Boil juices rapidly to reduce to about 1 1/2 cups. Add jalapenos, garlic, cumin, oregano and 1/2 teaspoon black pepper. Lock lid in place, bring to full pressure and cook 2 minutes. Reduce pressure quickly.
Puree liquid with hand blender or in standard blender or food processor. Add lemon juice and 1/2 teaspoon salt.
Remove meat from bones, discarding bones and fat. Coarsely chop meat with knife and then place meat in bowl. Add half of sauce to meat, mixing well. Add cayenne and cilantro to remaining sauce.
To make tacos, heat tortillas over griddle or low flame. Place about 2 tablespoons meat mixture in each tortilla. Top with remaining sauce, lettuce and tomatoes.
Makes 24 tacos. Each taco contains about: 132 calories; 101 mg sodium; 28 mg cholesterol; 4 grams fat; 14 grams carbohydrates; 11 grams protein; 1.14 grams fiber.