Take the meatballs out of the frying pan and set aside. Switch to low heat, add the onion, garlic and rosemary and stir for a minute. Add the flour and stir for one more minute. Add the right amount of beer and 25 ml water per person, stir until smooth. Add the meatballs to the pan and cook, covered, on low heat for 20 minutes. Bring to taste with salt and pepper.