Mix mustard, minced garlic and chopped thyme or rosemary in small bowl to blend.
Place lamb (rounded side up) on baking sheet lined with either foil or parchment.
Carefully trim the lamb fat by working a sharp knife from the meat down towards the bone. It's impossible to remove all of the fat but trim it as cleanly as you can. If buying the rack of lamb from a butcher, ask them to trim the lamb for you.
Spread mustard-herb mixture evenly over lamb.
Lamb can be defatted and coated with mustard-herb mixture up to 6 hours ahead. Refrigerate uncovered.
Preheat oven to 425°F roast setting (if you don't have a roast setting use the bake setting).
Before cooking, gently press gluten free bread crumbs onto mustard coating of lamb and drizzle with olive oil to help the bread crumbs brown while roasting.
Roast lamb for 25 minutes for medium rare. If you like it medium to medium-well, cook for an additional 2-5 minutes depending upon how well done you like your lamb.
Remove from oven and let rest for 5-10 minutes before slicing into chops.
Garnish with thyme or rosemary sprigs, if desired, and serve.