To a casserole pan add butter. When butter is hot stir in onions and saute until caramelized. Pour browned onions into a sieve over a bowl and let drain. Pour the fat from the onions back into the casserole and when the fat is hot add beef, and remove as slices become brown. Drain in the sieve with the onions.
In the same casserole, stir in the brown sugar and add the flour. When a roux has formed, stir in mustard and add beer slowly to deglaze the pan. Add beef stock and season with salt and pepper. Pour liquid into a bowl.
Layer the casserole with the meat and onions, making 3 layers of meat, sandwiching 2 layers of onions. Add a bay leaf and a teaspoon of mixed herbs on each layer of onions. Pour beef sauce back into the casserole. Add more beer to cover the meat if necessary. Bring casserole to a boil, cover and bake in a 300 degree oven for 3 hours. Skim off fat, season with salt and pepper and serve.