Cook the pasta according to the packet instructions.
Add the spinach to the last minute of the cooking time.
Meanwhile mix the blue cheese together with lemon zest and crème fraîche. Lightly toast (without oil) the walnuts, set aside, then roughly chop.
Drain the pasta and spinach and put back in the pan. Stir in the cheese sauce, then fold in the crayfish tails.
Divide the pasta between two plates, scatter the walnut bits and chilli flakes on top. You can crumble extra blue cheese on top if you like.