Wash the rosemary and thyme and dab dry. Pluck the thyme and rosemary leaves from the stems, except for a little to garnish. Peel the garlic and press it through a garlic press.
Wash the meat, dab dry and rub with salt, pepper, thyme, rosemary and garlic. Place the meat in the fat pan of the oven. Bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 2 - 2 1/2 hours.
Onions peel and quarter. After 30-45 minutes add onions to the meat and pour on 125 ml stock. From time to time ladle the meat and add some stock as required. Wash the asparagus, peel the lower third and cut off the woody ends.
Peel, wash and slice the carrots. Clean and halve the mushrooms. Steam asparagus and carrots separately in little boiling salted water for 8-10 minutes. Drain and keep warm. Heat the fat in a pan and fry the mushrooms until golden brown.
Season with salt and pepper and fold in the carrots. Wash the parsley, dab dry and chop finely, except for a little to garnish. Mix into the carrot vegetables. Keep leg of lamb warm and pour the roast stock through a sieve into a pot.
Dissolve the roast with water and also pour into the pot. If necessary, degrease the meat stock and bring to the boil. Mix sour cream and cornstarch and stir into the sauce. Bring to the boil again briefly and season with salt and pepper.
Arrange leg of lamb and vegetables on a plate. Garnish with herbs. Serve with extra sauce. Boiled potatoes taste good with it.