Preheat oven to 375 degrees. Cut the rack into two 4-rib pieces, and season the meat generously with salt and pepper.
Toast the mustard and fennel seeds in a large seasoned cast-iron (or ovenproof) skillet over medium heat, stirring constantly, until fragrant, 1 to 2 minutes. Transfer to a plate to cool slightly.
Heat oil in skillet until hot but not smoking. Place 1 piece of lamb in skillet, bone side down, and brown all over, 1 to 2 minutes per side. Transfer to a plate;
repeat with remaining piece of lamb. Roll meat in spices to coat (do not coat the cut sides on either end).
Return meat to skillet, and transfer to oven. Roast meat until an instant-read thermometer inserted into center (avoiding the bones) registers 135 degrees, or medium-rare, 18 to 24 minutes.
Remove meat from skillet, and let stand at least 10 minutes. Cut meat into individual or double chops, and cross the bones, if desired. Serve with herb oil.
It's important to let the meat "rest" for 10 minutes before serving (it will finish cooking and reabsorb juices).