Place mustard, garlic, rosemary, lemon zest and lemon juice in a bowl. Whisk in 2 tablespoons of olive oil. Season the lamb with salt and pepper. Rub marinade evenly over the lamb racks. Place in refrigerator overnight. Remove lamb about 30 minutes ahead of cooking time. Place sauté pan over mediumhigh heat. Add 2 tablespoons of olive oil to your pan. Sear lamb on all sides. Remove lamb from pan. Place bone-side down in a baking dish and roast for 12 to 15 minutes. Remove lamb from oven and let rest for 10 to 15 minutes on a cutting board. Serve with roast tomatoes and cheddar polenta cakes.