Preheat the oven to 450 degrees . In a medium bowl, toss the fennel and onion with 4 tablespoons of the olive oil and season with salt and pepper. Place the fennel and onion on a sheet pan.
In a large skillet, heat the remaining 2 tablespoons of olive oil over medium-high heat until almost smoking. Season the lamb with salt and pepper. Sear the lamb until well browned on all sides, 6 to 8 minutes. Transfer the lamb to a plate.
Combine the rosemary, mustard, horseradish and cheese in a bowl and stir until a paste forms. Place the rack of lamb on a sheet pan or roasting pan and spread the paste evenly over the lamb.
Place the vegetables on the lower shelf of the oven and the lamb on the upper shelf of the oven. Roast for about 15 minutes, then lower the heat to 400 degrees . Remove the vegetables from the oven and give them a toss. Move the lamb to the lower rack. Return the vegetables to the upper rack and continue roasting. The vegetables are done when they are tender and partly browned, 8 to 10 minutes. Roast the lamb for about 10 minutes longer, or until an instant-read thermometer inserted into the thickest part registers 130 degrees for medium-rare. Remove the lamb to a platter, cover loosely with foil and let it rest for about 10 minutes. Cut the rack of lamb between the bones into chops. Serve the lamb chops with the roasted fennel and red onion.