In a nonreactive, shallow bowl wide enough to hold the roast flat, pour the marinade over the meat. Add the bay leaves, the peppercorns, and the studded onion. Cover tightly and let stand in the refrigerator for at least 6 hours and up to 2 days, turning frequently and re-covering each time. Alternatively, place the marinade and the meat with seasonings in a plastic bag that closes with a zipper-like seal that will prevent any leakage. Simply turn the bag from time to time to insure that the meat is evenly marinated.

An hour before you're ready to cook the roast, remove the meat from the refrigerator and prepare a fire in your charcoal grill. When charcoals are covered with gray ash and fire is moderately low, place the roast on the hot grill. Season lightly with salt and pepper. Strain the marinade into a saucepan and bring to a boil, reduce heat, and keep hot. Cook roast covered with the vents open until well browned on the outside and rosy pink on the inside, about 15 minutes each side, basting from time to time with the marinade if desired. Flames that result from fat dripping on the hot coals tend to bum the outside of meat, keeping the inside uncooked. Check often and douse with water as needed to prevent unwanted flames. Let roast stand for 10 minutes. Slice thinly across the grain and serve with the marinade sauce on the side.