Take the meat out of the fridge at least 30 minutes in advance.
Preheat the oven to thermostat setting 6/180 °C.
Sear the racks quickly on all sides on a high heat in a frying pan with a little oil. Place them in a roasting tin, season and roast for 10 minutes in the oven.
Meanwhile, toast the slice of bread and break it into pieces. Place in the blender and add the pressed cloves of garlic and parsley. Blend until you have a coarse but even breadcrumb mixture.
Mix the mustard with the white wine and herbes de Provence.
Take the lamb out of the oven and turn down the heat a little (thermostat setting 5-6/170 °C). Smear the racks of lamb with the mustard-based mixture then press on the breadcrumbs so that the meat is entirely covered. Cook for a further 15 to 20 min in the oven, to suit your preference.
Cover the dish with tinfoil and allow to rest for 5 min. Slice the racks between the bones and serve immediately.
You can replace the parsley (all or part) with a choice of other fresh herbs: tarragon, basil, chives or sage.