Preheat the oven to 450 degrees F. Line a sheet pan with aluminum foil.
In a medium bowl, crumble the goat cheese with a fork. Add the panko, garlic, rosemary, thyme, 1 teaspoon fleur de sel and 1 teaspoon pepper. Drizzle with 3 tablespoons olive oil and stir to moisten the crumbs. Set aside for a few minutes or cover and refrigerate.
Remove the racks of lamb from the refrigerator 30 minutes before you plan to cook them. Place the lamb, fat-side up, on the prepared pan. Sprinkle with 2 teaspoons fleur de sel and 1 teaspoon pepper. Roast for 12 minutes exactly. Remove from the oven and, working quickly, use a knife to spread the mustard on the top of the lamb. Spread the crumb mixture evenly on the mustard, pressing gently to help the mixture adhere. Return the lamb to the oven right away and continue roasting for another 12 to 18 minutes (depending on the size of the lamb), until the crumbs are golden brown and the meat is 120 degrees F for rare or 125 degrees F for medium rare. (Insert an instant-read thermometer horizontally through the meat.)
Cover loosely with aluminum foil and allow the lamb to rest for 8 minutes. Cut the racks into single or double chops and serve hot.
"Frenched" lamb chops look more elegant. When you ask your butcher to french the lamb, he will trim the excess fat from the bones.