Cook the pasta according the instructions on the packet. Drain and keep warm (keep a glassful of the cooking water).
Meanwhile slice the onion and chop the garlic. Heat a frying pan and dry-fry the nuts then transfer them to a plate.
Put the pan back on the stove, heat a spoonful of oil, turn down the heat and gently sauté the onion and garlic. Add the kale ( and chilli flakes if used) and a drop of the pasta’s cooking water. Put the lid on and cook for 3-5 minutes until the kale has wilted.
Crumble in the cheese, let it melt, then stir in the crème fraîche.
Add the pasta, stir for a minute or so to warm through. Crumble the walnuts and some more cheese on top and serve.