Season: Sprinkle lamb rack with salt and pepper.
Sear: Heat 1 tbsp oil in a heavy based skillet over high heat. Sear lamb rack all over until nicely browned – including each short end – about 1 1/2 minutes on each side. It will be fully raw inside, but that's OK. Transfer to plate and let cool for 5 minutes, uncovered.
Mustard Spread: Mix dijon, rosemary and garlic in a small bowl. Then add 3 teaspoons of egg, and mix again.
Making the crumb: Mix Crumb ingredients EXCEPT butter in a bowl. Then use a fork to stir through butter. Spread on a dinner plate (large enough that fits lamb).
Crumbing the lamb: Spread the underside of the rack with mustard mix. Press into breadcrumb mixture, then spread mustard mix over the other side as well as each end. Then press those sides into the breadcrumb mixture.
Transfer lamb to rack set on a tray. Surround with parboiled vegetables if using – but don't crowd the lamb.
Roast: Frenched racks for 20 minutes, or untrimmed racks for 30 – 35 minutes, or until internal temperature registers 60°C/140°F (for medium rare).
Rest meat: Transfer lamb to a cutting board, loosely cover with foil. (If you roasted veg, leave in tray or pan and keep warm in turned off oven).
Carve: Rest 5 minutes then slice carefully using your hands to hold the crumb gently in place where you are cutting using a very sharp knife. If your service allows it, it's best to cut 2 or 3 bones together (ie. a double/triple cutlet portion), because the crumb stays on better. Slicing single cutlets is super hard (sadly)!
Serve as-is (no sauce), with Creamy White Wine & Mustard Sauce, or Pea Puree.