In a large shallow bowl or container. Pat the lamb dry, then place into the bowl or container with the marinade and turn and use your hand or a basting brush to make sure the marinade coats the lamb completely. Refrigerate, and marinate for about 8 hours, if possible turning the lamb and re-brushing on the marinade once or twice during the marinating time.
Preheat the oven to 400 °F. Remove the lamb from the marinade and allow to come to room temperature, about 30 minutes. Add the breadcrumbs to the remaining marinade, stir to combine well, and press it into the meaty part of the lamb racks to coat in a thin layer.
Place the chops curved bone side up in a roasting pan or rimmed baking sheet and roast for 22 to 24 minutes for rare (internal temperature of 135°), or about 24 to 28 minutes for medium rare (140°F internal temperature). Transfer the lamb to a cutting board and let sit for 10 minutes.
While the lamb is cooking and resting, make the Orange Thyme Béarnaise sauce: In a small bowl combine the mayonnaise, zest and juice of the orange, thyme, olive oil, and salt and pepper.
Slice the racks into individual chops, place on a platter (or individual plates), and garnish with thyme sprigs. Serve with the sauce.