Preheat the oven to 200°C.
Season the lamb with salt and pepper and sear in 3 tablespoons of hot olive oil until browned.
Place lamb on a wire rack to cool and toast breadcrumbs with remaining oil in the oven until brown.
Reduce lamb stock to sauce consistency and stir in herbs, mustard, and 15g of butter.
Boil potatoes until tender, سپس drain and crush with melted butter and spring onions.
Mix the two mustards together, brush onto lamb loins, and press on the toasted herb breadcrumbs.
Roast the lamb at 200°C for 4 to 8 minutes depending on preferred doneness, then rest.
Sauté the blanched vegetables and mushrooms in a little oil and season with salt and pepper.
Serve sliced lamb over potatoes with vegetables and sauce, garnished with parsley.