Preheat the oven to 450F.
Combine the breadcrumbs, garlic, mint, thyme, rosemary, salt, and oil in a bowl. Stir to combine.
Season the lamb with salt and pepper and rub with the mustard. Rub the breadcrumb mixture on both sides until covered.
Roast the lamb for 20 minutes or until the internal temperature reads 125F for rare to medium rare. Let the lamb rest for 15 minutes before slicing.
Meanwhile, place the chopped turnips in a pot of cold water. Bring to a boil over high heat and cook until tender. Drain. Heat the cream and butter together until warm.
Mash the turnips with a potato masher. Add the warm cream and butter and mix to combine. Season generously with salt and pepper.