This deeply comforting dish feels quite indulgent, but actually, you're getting off lightly here. The tender lamb backstrap is lean, you have a good helping of pan-wilted greens, and the mash is carb-light with cauliflower and butter beans. OK, so there is a butter-rich sauce, but we're willing to overlook that one if you are.
Cut cauliflower into small florets. Place in a saucepan with milk and fill with cold water to just cover. Cover with lid, place saucepan over medium heat and bring to just below the boil. Remove lid and cook, uncovered, for 18–20 minutes or until cauliflower is very tender. Drain. While cauliflower is cooking, continue with step 2.
Trim beans. Remove stems from kale and tear leaves into bite-sized pieces. Pick thyme leaves from their stems. Peel and crush garlic.
Rinse and drain butter beans. Once cauliflower is cooked and drained, add butter beans to pan and use a stick blender to blend cauliflower and beans until smooth. Add parmesan, salt and pepper and mix to combine. Set aside and keep warm until ready to serve.
Season lamb with salt and pepper on both sides. Heat 2 teaspoons oil in a frypan over medium-high heat. Add lamb and cook for 3–3½ minutes each side for medium or until cooked to your liking. Remove pan from heat, add butter, thyme and mustard and turn lamb to coat in melted butter. Place lamb on a plate, pour over butter, cover with foil and rest for 5 minutes.
Wipe out pan and return to medium-high heat. Add remaining oil, garlic, green beans, kale and 20ml (1 tablespoon) water to pan. Cover with a lid and cook for 3 minutes or until greens are tender. Season with salt and pepper.
Slice lamb. Divide cauliflower mash and greens between plates and top with lamb. Spoon over any resting juices to serve.