1. Preheat the oven to 375°F.

2. Rub the racks of lamb with the olive oil, garlic, rosemary, and salt and pepper.

3. Place the racks of lamb in a roasting pan and bake in the preheated oven for 25-30 minutes, or until the lamb is cooked through.

4. Remove the lamb from the oven and allow to rest for 10 minutes before carving.

5. Meanwhile, in a small saucepan, melt the butter and add the beef or veal stock. Simmer for 10 minutes, or until the sauce has reduced by half.

6. Carve the lamb into individual chops and serve with the rosemary jus.