Combine garlic and 1 1/2 teaspoons of the oil; brush all over buffalo 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 a cutting board and let rest 5 minutes. 

Slice thinly against the grain, transfer to a platter or plates, and surround with the onions.

 If you like, sprinkle a few more pinches pink salt around edges of plates or platter.