Wash the meat, dab dry and bind together with kitchen string to form a comb. Peel garlic and press it through the garlic press or dice it very finely. Rub lamb comb with garlic, salt, pepper and dried herbs.
Put oil in the fat pan of the oven and place the meat on top. Bake in the preheated oven (electric: 200-225°C/ gas: level 3-4) for 35-40 minutes. Wash the potatoes thoroughly and cook in plenty of boiling salted water for about 20 minutes.
Wash and clean the beans and break them through once or twice. Cook the beans in little boiling salted water for 10-15 minutes. Clean, wash and quarter the mushrooms. Peel and finely chop the onion. Drain potatoes, rinse, peel and quarter them.
Heat 30 g fat in a pot and sauté the flour in it until golden brown. Deglaze with stock and milk, bring to the boil while stirring and simmer for about 5 minutes. Wash the rosemary, pluck the needles (approx. 1 level tablespoon) and chop.
Add half of the chopped rosemary to the sauce. Add the egg yolk to the sauce and season to taste with salt and pepper. Add potatoes to the hot sauce. Leave the lamb comb wrapped in aluminium foil to rest for 5-10 minutes.
Heat the rest of the dripping in a saucepan. Sauté the onion and mushrooms in it, salt, then toss the drained beans in it. Arrange the meat and vegetables on a preheated plate and garnish with a small bunch of fresh herbs.
Place the béchamel potatoes in a bowl and sprinkle with the remaining rosemary.