Rub the leg of goat generously with salt and pepper. Melt the butter in a roasting pan, and sear the leg of young goat in it until well browned all over, then take it out of the pan.
Sautee the onions in the pan juices, add the diced tomatoes and thyme, then place the leg of goat on top.
Pour 1 cup of beer over the leg, then roast in a preheated oven at 350 degrees F (180 C) for 1-1/2 hours. Baste with the remaining beer during this time. Take the leg out of the roasting pan and keep it warm.
Strain the pan juices, and bring to a boil. Continue boiling until slightly reduced, then season to taste with thyme, salt and pepper.
Serve with mashed turnip or red cabbage with apples, and potatoes boiled in their skins.