In a bowl, whisk together all of the ingredients, except the onions, Essence, and peppercorns.
Place the steaks in a large sealable bag with the marinade. Seal and refrigerate for at least 12 hours, and up to 36 hours.
Preheat a grill.
Remove the steaks from the marinade and pat dry. Season both sides with 1 tablespoon of the Essence and the peppercorns. Pour the marinade into a small saucepan and bring to a simmer. Simmer until thickened and reduced by 1/3 in volume, about 5 minutes, stirring occasionally. Remove from the heat.
Sprinkle the thick onion slices with the remaining 1 tablespoon of Essence.
Grill the steaks until medium-rare, about 6 minutes per side. At the same time, grill the onions (as though grilling a patty, so the rings stay together) until golden and char-marked by the grill, turning once.
Remove the steaks from the grill and top with the grilled onions. Serve with the reduced marinade as sauce.