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Heat oven to 200C/180C fan/ gas 6. Heat 1 tbsp oil with the butter over a medium heat. Add the bay leaf, rosemary and finely sliced leek, season and cook for a few mins until the leek has softened.
Add the flour and cook for 1 min more until the roux smells nutty. Take off the heat, slowly pour in the milk and whisk out any lumps.
Return to the heat and whisk slowly until thick and smooth, about 5 mins. Season well with grated nutmeg, then add 3/4 of the cheddar and parmesan, cooking until they are melted. Set aside.
In another pan, heat 2 tbsp oil over a medium heat and fry the garlic and chilli for a couple of mins until aromatic. Add the rest of the leeks, season and fry until softened, then add the mixed greens and season a little more. Cook, stirring, until the greens have wilted, about 4 mins, then pour in the wine and cook until it evaporates.
Oil a medium roasting tin (about 20cm square) and tip half the greens into the tin. Dot half the walnuts over the top, and nestle in a third of the artichoke hearts. Dot over half the ricotta. Remove the bay leaf from the leek bechamel and pour over a third of it. Top with three lasagne sheets to cover everything in a at layer. Repeat the process, then cover the top layer of lasagne sheets with the remaining bechamel and artichokes and scatter over the remaining cheese. Bake in the oven for 30-40 mins, until golden and bubbling. Rest for 10 mins, then serve with a sharply dressed grated carrot and beetroot salad.