The day before, wash the meat and pat dry. Peel and chop onion and garlic. Lightly crush the juniper. Put meat with buttermilk, juniper, bay leaf, onion and garlic in a large bowl and cover and chill overnight.
The next morning remove meat from the marinade and pat dry. Sieve the marinade, catch the spices and onions and set aside. Rub the meat with salt and pepper and tie it into shape with kitchen string.
Wash the thyme and shake dry. Heat the oil in a frying pan. Fry the meat thoroughly all around. Add onion-spice mixture and fry briefly. Add 3⁄4 l water, thyme and roasted onions.
Cover and braise in a hot oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer) for about 3 hours. Turn once in between.
Peel and wash all potatoes and cut them into approx. 1 cm thick slices. Take the meat out of the roaster and sieve the frying stock. 1⁄4 Measure l and put it back into the roaster with the onion mixture and 100 g cream.
Spread potatoes in it. Season with salt and pepper. Place the roast on top. Cook open at 200 °C (convection oven: 175 °C/gas: see manufacturer) for approx. 40 minutes.
Put the rest of the frying stock and 200 g cream in a pot and bring to the boil. Mix the starch with a little water until smooth. Stir into the stock and simmer for about 1 minute. Season the sauce with salt, pepper and sugar. Remove the leg of lamb and arrange it on a plate with the potatoes.
Add the sauce.