Step 1 Preheat the oven to 180 degrees Step 2 Cut the green part off the leek, don’t throw it away, it can be used in other recipes! Wash the white part well to remove any grit and then thinly slice. Put the sliced leek into a small baking tray. You want to choose a small baking dish to keep everything snug and the lentils juicy. Step 3 Peel the garlic and thinly slice, and add to the leek. Drizzle with the olive oil, and then add the salt, pepper and thyme and mix together with tongs. Step 4 Drain and rinse the lentils and add to the tray. Thinly slice the mushrooms and add to the tray. Mix again. Step 5 Add the beef stock and dot the butter across the top of the vegetable, lentil mix. Cover the tray with foil and cook for 20 minutes. Step 6 While the base is cooking. Heat a fry pan, add a little oil and sear the lamb for 3 minutes on both sides or until beautiful and brown. Step 7 Remove the tray from the oven and carefully remove the foil. Taste, adding more salt or pepper if needed. Then place the lamb in the middle of the tray, nestled into the vegetables and lentils. If the lentils are looking dry add a little more stock, water or a splash of wine. Step 8 Return to the oven for another 30 minutes, or until the lamb is perfectly cooked. Check the authors notes below for lamb cooking times and weights. Step 9 Remove from the oven, take the lamb off the lentils and transfer to a plate or board, loosely covered in foil. Let sit for 5 minutes or so. While the lamb is resting stir a few big handfuls of baby spinach through the lentil mix. The heat will wilt the spinach. Thinly slice the lamb and return to the top of the lentils. Step 10 Serve warm.
