Method:
Combine garlic and 1 1/2 teaspoons of the oil; brush all over bison steaks. Let steaks sit at room temperature as you prepare a grill for medium-high heat cooking. In a small bowl, combine vinegar, rosemary, fine sea salt and remaining 1 1/2 teaspoons oil.
Sprinkle steaks with pink salt and pepper; grill, turning once or twice, until they reach desired doneness, 8 to 10 minutes for medium-rare. Brush onion slabs with vinegar mixture and grill alongside steaks, turning occasionally and basting with vinegar mixture, until browned and tender, 10 to 12 minutes.
Transfer steaks to cutting board and let rest 5 minutes. Slice thinly against the grain, transfer to platter or plates, and surround with the onions. If you like, sprinkle a few more pinches pink salt around edges of plates or platter.