Preheat the oven to 350°F (175°C).
Mix the minced garlic, herbs, and olive oil together. Gently toss into the breadcrumbs. Add the salt and pepper. Set aside.
Bring the lamb racks to room temperature; otherwise it will affect the cooking time. Score the fat on the racks into large diamond shapes. Cut the 2 remaining garlic cloves in half and rub over all sides of the fat and meat. Sprinkle all sides of the meat liberally with salt and pepper. Heat the oil and butter in a frying pan and sear the meat on all sides for about 1 minute per side. Wrap the bones in tin foil so they do not darken. Transfer to a baking pan and roast for 15 to 16 minutes.
Remove from the oven and spread ¼ cup (60 mL) of mustard on each rack. It seems like a lot, but don't worry--it needs it. Pat ½ cup (120 mL) of the breadcrumbs firmly on top of the mustard on each rack. Return to the oven for 10 minutes. This will give you rosy lamb. Cook for another 4 minutes for medium. Let the lamb rest for at least 10 minutes before cutting. Cut between every second bone.