Step 1: Prepare the Lamb - Preheat your oven to 400°F (200°C). Trim any excess fat from the racks of lamb.
Step 2: Make the Pepper Crust - In a food processor, pulse the whole black peppercorns until finely ground. Mix with breadcrumbs, garlic, butter, rosemary, thyme, lemon zest, and salt in a bowl.
Step 3: Apply the Dijon Mustard - Rub the Dijon mustard all over the lamb racks.
Step 4: Coat with the Crust Mixture - Press the pepper crust mixture onto the lamb racks evenly.
Step 5: Roast the Lamb - Place the racks in a roasting pan, bone side down. Roast for about 25-30 minutes for medium-rare. Check the temperature with a meat thermometer.
Step 6: Prepare the Horseradish Crème Fraiche - Mix crème fraiche with horseradish and a pinch of salt until smooth.
Step 7: Rest and Serve - Let the lamb rest for 10-15 minutes, then slice and serve with horseradish crème fraiche.