Roughly dice the beef and the onions. Sear both in clarified butter, season with salt and pepper. Add the brown sugar. Then slowly pour in the beer, it should be more than just covered. So take a lot of beer.
Brush the rye bread (large slices) with the two types of mustard. When the beer boils up, place the bread on top with the mustard side down and put the stove on a low flame. Add a few bay leaves.
Put the lid on, but don`t close it, it should stay open a little. Let simmer on a low flame for 3 - 4 hours. Stir occasionally and press the bread down.
Season to taste, if necessary add more sugar. The flesh almost falls apart after time. I eat this dish with red cabbage and boiled potatoes.