Put oven rack in middle position and preheat to 400 fahrenheit.
Stir together breadcrumbs, parsley, mint, rosemary, salt, and pepper in a bowl, then drizzle with 2.5 tbsps oil and toss until combined well.
Season lamb with salt and pepper. Heat remaining tbsps oil in a large heavy skillet over moderately high heat until hot but not smoking, then brown lamb 1 rack at a time, turning once, about 4 minutes per rack. Transfer to a 13 by 9 by 2-inch roasting pan, arranging fatty sides up.
Spread fatty sides of each rack with mustard. Divide bread crumb mixture and pat each portion over mustard coating on each rack, gently pressing to adhere.
Roast lamb until thermometer inserted diagonally 2 inhes into center (do not touch bone) registers 130 fahrenheit (for medium-rare), 20 to 25 minutes, and then transfer to a cutting board.
Let stand 10 minutes, then cut into chops.