Cut the meat with the tip of a knife and insert some rosemary needles, then sprinkle with black pepper, massaging the meat to make it stick well.
Heat a few tablespoons of oil in the pressure cooker, add the chopped onion and some rosemary needles, then brown the meat on all sides. Moisten with a large glass of beer, close the lid and cook for 30 minutes from the whistle.
Peel the potatoes and cut them into pieces. Drain the pan, add the potatoes and remaining beer, then close the lid again and cook for 10 minutes from the whistle. Let stand for about ten minutes then open the pot, cut the meat into slices and serve with potatoes.