Preheat oven to 450℉. Mix the parsley, mint, breadcrumbs, salt and pepper in a bowl. Drizzle with olive oil and toss to combine. Set aside.
Lightly score the fat side of your rack of lamb by creating an "X" pattern across the top. Dragging your knife gently across the fat from end to end and creating a superficial cut. Generously season both sides with 1½ tsp of both salt and pepper. Massage into the lamb.
Add 1 tbsp butter to a hot cast iron skillet over medium high heat until melted and frothy. Lay the lamb racks fat side down into the hot skillet and let sear for about 1.5 mins before flipping and repeating the sear for another 1.5 mins.
Remove the lamb racks from pan and brush (or use the back of a spoon to spread) 1 tbsp of dijon mustard on one side of the ribs. Add a generous few tbsp of the herb mixture to the mustard glazed lamb, patting the crust in to stick. Repeat with the remaining mustard and herb crust.
Place the racks in the same cast iron pan and place in the oven to bake until internal temperature reaches 135℉, about 15-20 minutes.
Remove from oven and let rest 10-15 minutes before slicing along each rib bone for serving.