Sprinkle the lamb ribs with salt and pepper. In a large non-stick skillet over medium-high heat, sear the lamb ribs until browned, about 2 minutes per side. Transfer to a roasting pan and let cool slightly.

Preheat the oven to 400°F. In a small bowl, combine the California walnuts, thyme, rosemary, garlic, salt, and pepper with a fork. Add the oil and stir until well combined.

Using a brush, spread the mustard all over the lamb. Press the walnut mixture into the mustard to form a crust. Roast the lamb in the preheated oven until medium-rare, about 15 to 20 minutes. Remove from the oven, cover with aluminum foil, and let rest for 5 to 10 minutes. Slice between the bones and serve 2 or 3 chops per person.