Place the beef in a shallow dish. Mix the bay leaf, olive oil, mustard, lemon juice, garlic and peppercorns together and pour ove the beef. Cover and refrigerate for 8 hours. Baste 2-3 times while chilling.
Heat the vegetable oil in a roasting pan in a preheated oven at 400 degrees F for 3-4 minutes. Remove the meat from the marinade, place on a rack in the roasting pan, baste with the hot oil, and cook for 30-45 minutes, until done as desired. Leave to cool.
Meanwhile, boil the potatoes until tender, cut in half and toss with 2 T dressing. Leave to cool.
Slice the beef and arrange in overlapping slices round a flat serving plate. Brush with cool gelatin, leave to set. Decorate with pepper diamonds, brush with gelatin and leave to set.
Mix the avocado, remaining dressing, potatoes and olives together. Arrange on a serving plate, garnish with mint sprigs, and serve with beef.