In a flat dish, combine the butter, parsley, marjoram, rosemary, and breadcrumbs. Mix well.
Put the dijon mustard into a small bowl. Set both the crumb mixture and mustard, along with some tongs, and a basting brush beside the grill.
Preheat one side of the grill to high, about 500°F.
Using a sharp knife, score the fat on the lamb racks in a diamond pattern, making sure not to touch meat.
Lay the racks, meat side down, on the hot side of the grill to sear. Keep the bones away from the heat and leave them to form a nice deep brown crust, about 5 to 7 minutes. Do not allow the racks to burn.
Using your tongs, lift each rack by the bones and brush the seared meat with a layer of mustard. Then, dip them into the crumb mixture and press down so that a thick coat of the mix will stick to the rack.
Set the coated racks, bone side down, on the “cool” side of the grill near the heat, but have the rib ends pointing away from the “hot” side of the grill. Close the lid and roast the lamb until the crust has browned and and the rack is medium-rare to medium. This takes about 15 to 20 minutes. A meat thermometer will read 130°F to 135°F.
Remove the racks to a platter and allow them ro rest uncovered, (This prevents the crust from getting soggy) for 3 to 5 minutes.
Carve the racks between every, or every other bone and arrange on a platter in a cluster. Serve immediately with seasonal vegetables and some potatoes.