Boil water, add peppercorns and fillet and cook slightly. After 1 1/2 hours, add root vegetables, onions and leek and simmer for another hour.
Remove meat and season broth. Leave meat to cool in fridge then finely slice across the grain.
For marinade: First mix mustard, balsamic vinegar and lime juice in broth. Slowly add oil and whip until creamy. Season with salt and pepper. Pile meat slices on top of each other in container, pour marinade over them and leave to steep overnight.
Arrange slices on plate, garnish with onion rings and salad and sprinkle with dressing.