Clean, wash and roughly dice the soup vegetables. Wash the roast and dab dry. Rub with salt and pepper and place on the fat pan of the preheated oven (electric cooker: 200 °C/ gas: level 3).
Add soup vegetables. Pour hot clarified butter over them. Braise for about 1 1/4 hours. After 30 minutes pour 1/2 litre of water over it. Add 1 teaspoon of mustard seeds. For the crust, chop chervil except for something to garnish.
Mix mustard, egg, breadcrumbs and chervil. Spread on the roast for the last 1/4 hour. For the vegetables, clean and wash the spring onions. Blanch in broth for about 5 minutes, drain.
Swivel in hot fat before serving. Pour the stock through a sieve. Fill up to 1/2 litre with cream and possibly some water. Add remaining mustard seeds. Let cook for about 5 minutes. Stir starch with a little cold water until smooth.
Thicken the sauce with it, bring to the boil again. Season to taste with salt and pepper. Arrange the roast with some sauce and the vegetables on a plate. Garnish with retained chervil. Remaining sauce extra is enough.