Baste occasionally with reserved marinade. Spoon hot marinade over steak, and sprinkle with sesame seeds as desired. Serve with rice, asparagus, a mixed-greens salad and sorbet. Preheat oven to 350 degrees. Brown beef with onion, salt and pepper.
Stuff shells with meat mixture.
Pour remaining sauce over shells and bake uncovered 30 minutes. Sprinkle remaining onion rings and cheese over top. Bake 5 minutes or until cheese is bubbly.