Pulse all ingredients into a food processor until everything is finely chopped and well-combined.
Rack of lamb:
Season all sides of the lamb with salt and pepper. Heat a frying pan (ovenproof) with olive oil until hot. Add the racks of lamb (start with fatty side first) and brown on each side (1-2 min. by side until brown).
Transfer the racks of lamb on a baking sheet, bones down. Let cool a little bit then generously brush each fatty side of the lamb with the mustard. Divide the herb crust in two, coating each rack with herb crust. Gently press to adhere herb crust over the mustard.
Put the two racks back in the frying pan (fatty side up), drizzling a bit more oil if you want. Roast in the oven for 20-25 min. or until a thermometer inserted in the thickest part of lamb registers 130°F for medium rare. Let lamb rest at least for 10 minutes before slicing into chops. Cover with aluminum. Transfer the meat on cutting board. Cut into chops and serve.
Mashed butter bean:
While the lamb is cooking, in a food processor, add the butter beans, garlic, anchovies, rosemary, lemon juice, and olive oil. Pulse until smooth.