Soak the römertopf for 10 minutes. Wash the leg of lamb and pat dry. Season with pepper, salt and chilli powder, then add to the Römertopf. Clean and quarter the carrots and peppers and spread around the leg of lamb. Peel the onions, cut into large rings and spread on the leg of lamb. Squeeze the garlic and mix with the tomatoes and their juice and pour the mixture over the onions. Place fresh thyme sprigs on top or, if not available, sprinkle with thyme powder.
Close the Römertopf and place in the cold oven. Set the oven to 180-200 degrees and cook the leg of lamb in it for about 4 hours. Open the Römertopf 15 minutes before the end of the cooking time.
Potatoes or pasta can be served with a mixed salad.