Heat the oil in a pan and lightly fry the onion and garlic, stir in the mustard and deglaze with 100 ml of beer. Remove from heat, add thyme, pepper and let cool. Then add the rest of the beer and pour the marinade over the meat. Depending on the size of the meat, let it stand for a few hours or large roasts for up to 2 days.
If you like it spicy, you can add a chilli pepper to the marinade.