Melt 75g of the butter in a heavy-based saucepan over a medium heat. Add the flour and cook out for 1–2 minutes, then slowly add the milk, stirring as you go. Cook for 8–10 minutes, stirring continuously without letting the sauce boil too hard, until thickened. Once thickened, add the mustard and nutmeg.
Preheat the oven to 180°C/160°C fan/Gas mark 4 and grease a lasagne dish with butter.
Melt the remaining 25g of butter in a saucepan, add the leek and cook for 2–3 minutes to soften a little. Season with salt and pepper, then add the spinach leaves (a handful at a time is easier) and the garlic and cook for a further 2–3 minutes, until the spinach has wilted. Drain off the excess liquid.
Spread some of the sauce over the bottom of the dish and place a layer of lasagne sheets on top. Top with some of the leek and spinach mixture, then some smoked salmon. Sprinkle over some cheddar and parmesan. Repeat the process, finishing the top layer with sauce, a few pieces of salmon, and grated cheese. Bake for 45 minutes, until golden and the lasagne is tender. (To check if the lasagne is cooked, insert a fork – if there’s no resistance, it’s ready.)
Sprinkle the dill over the top and serve with a fresh green salad.
Tom Kitchin’s Salmon and Spinach Lasagne is taken from the cookbook, Chefs at Home. It is a compilation of simple dishes from the nation’s most renowned chefs, which they prepared at home during lockdown when there were limited ingredients available. 100% of all royalties go to Hospitality Action.