فخذ خروف مشوي بالزبدة

1/2 onion
4 cloves garlic, peeled
2 tablespoons fresh rosemary leaves, or 1 tablespoon dried
Zest of 1 lemon
1 tablespoon apple cider vinegar
2 tablespoons extra virgin olive oil
Kosher salt and freshly ground black pepper
1 boneless leg of lamb, 5 to 6 pounds, butterflied