Make the red wine sauce. Combine 2 cups red wine, 2 cups chicken stock, 3 sprigs rosemary and3 cloves smashed garlic in a large saucepan. Bring to a boil over high heat, then continue cooking until liquid has reduced to about ¾-1 cup, about 25 minutes. Strain sauce, then set aside.
Bring the lamb to room temperature. About 30 minutes before cooking, remove lamb from refrigerator to come to room temperature. Place on a plate and pat dry with a paper towel.
Season the lamb on all sides. Preheat oven to 425°F (220°C). Set aside 1 tsp Kosher salt and ½ tsp black pepper, then use remaining salt and pepper to season lamb on all sides.
Sear the lamb. Heat oil in a large cast iron or heavy bottom skillet. When hot, place lamb meat side down and cook until browned on all sides, about 10 minutes. Remove and let cool slightly.
Make the goat cheese breadcrumb mixture. Place 1 cup panko breadcrumbs, 4oz goat cheese, 1 Tbsp rosemary, 2 Tbsp garlic, remaining Kosher salt and 1 tsp black pepper in a small bowl. Use a spatula or spoon to combine ingredients until well incorporated.
Cook the rack of lamb. Use a pastry brush or spoon to spread 1 tsp Dijon mustard evenly on meat. Use hands to press goat cheese-panko mixture firmly on top of rack of lamb. Return lamb to cast iron skillet. Surround lamb with 2 cups of red grapes and a few sprigs of fresh rosemary. Cook for 25 minutes, or until lamb has reached an internal temperature of 120-125°F for medium rare (49-51°C).
Rest, then cut and serve. Cover loosely with aluminum foil, then allow meat to rest 8-10 minutes. Lamb will continue cooking and internal temperature will rise 5-10 degrees more. When ready, cut into single or double chops and serve with a sprinkle of flaky sea salt, roasted grapes, and red wine sauce.