Wash the meat, pat dry and fry in 2 tablespoons of hot oil all around for about 10 minutes. Season with salt and pepper. Take out. Roast mustard seeds in the frying oil. Pour on vinegar. Stir in mustard and remaining oil.
Drain the bean seeds. Wash and halve or quarter the tomatoes. Fold both under the vinagrette. Cut the lamb open and serve. Add the vinagrette. Sprinkle with tarragon