An elegant dish that looks like a lot of work but is actually pleasantly simple to make for a special weekend dinner.
Remove the rack of lamb from the fridge about 1 hour before cooking, to allow the meat to come to room temperature. (This allows for more even roasting). Trim excess fat from the lamb so that an even, thin layer of fat is remaining. Season with salt and pepper to taste.
Preheat oven to 400°. Prepare a baking dish or roast pan just large enough to hold the rack of lamb. Add potatoes to the dish, pilling them to one of the long sides of the pan, and set aside.
In a small bowl combine all of the Herb Crust ingredients. Set aside.
Heat olive oil over high heat in a large sauté pan. Add lamb rack, fatty side down, and brown until fat is rendered in the pan, and the fat on the lamb is golden and crisp. Use tongs to position the rack to evenly brown all sides of the meat. All together the browning should take about 5 minutes.
Transfer lamb to a plate. Spoon pan drippings into prepared baking dish over the potatoes; stir to coat. Add salt to taste over the potatoes. Position the rack of lamb into the dish fatty side up, with the meaty part of the rack touching the bottom of the dish, and the bone tips arching over the potatoes. Spread Dijon mustard over the fatty side of the rack, and also the sides where the meat is exposed. Pat the reserved breadcrumb mixture onto the mustard to form a crust. Insert an oven thermometer if you have one.
Add the lamb to the oven and roast until the meat is medium rare (130°). This should take approximately 20–25 minutes. Remove roast from oven, place onto a cutting board and tent loosely with foil. Allow to rest for 10 minutes before cutting into chops and serving with the potatoes.