Leek carbonara with mushrooms and sage

1. Bring a large pan of salted water to the boil. In another pan, heat 15g butter over a low heat until bubbling; add the leeks, garlic and a pinch of salt. Cook gently for 15 minutes, until soft, then increase the heat a little. Shred ½ the sage leaves and add to the pan with the mushrooms. Cook for 2-3 minutes, stirring, until light golden. 

2. Meanwhile, cook the spaghetti according to pack instructions, then drain, reserving a mug of cooking water. Stir the pasta through the leek mixture. Take off the heat; set aside for 2 minutes. 

3. While it cools, add the remaining 15g butter to the empty pasta pan and return to a medium heat. When the butter starts foaming, add the remaining sage leaves and cook for about 1 minute, until golden and crisp; set aside. In a medium bowl, lightly beat the egg, then finely grate the cheese and stir it in. Add 2 tbsp of the pasta water, then pour over the spaghetti. Toss thoroughly until all combined in a silky sauce, adding more pasta water if needed. Season and serve topped with the crispy sage leaves.