How to Make It
Light a grill or preheat a grill pan. Brush the onions with oil and season with salt and pepper. Grill over moderate heat, turning once, until lightly charred and tender, about 7 minutes.
In a saucepan, bring the vinegar and maple syrup to a boil. Add the onions and simmer over moderately low heat until tender, 8 minutes. Transfer to a bowl; let cool. Stir in the thyme. Season with salt and pepper.
Light a grill or preheat a grill pan. Brush the steaks with oil and season with salt and pepper. Stand the steaks on their fatty edges (bone side up) and cook over moderate heat until the fat has started to render and is lightly charred, 5 minutes. Lay the steaks flat. Grill, turning often, until an instant-read thermometer inserted in the thickest part registers 120°, 12 to 15 minutes. If using a grill pan, carefully blot excess fat with paper towels to prevent flare-ups. Transfer to a cutting board; let rest for 10 minutes. Leave the grill on.
In a bowl, blend the butter, rosemary, thyme and garlic. Brush the herb butter on the steaks and grill over high heat, turning once, until sizzling, 1 minute per side. Transfer to a carving board. Slice the steaks and serve with the onion jam.