Preheat the oven to 200˚C / 180˚C fan / gas mark 6.
For the sauce:
Melt Lurpak® Slightly Salted Butter in a saucepan, add the shallots and bay leaves and cook for 10 minutes until softened. Stir in the flour and cook for 30 seconds before gradually whisking in the milk off the heat at first, followed by the stock on the heat, stirring continuously. Simmer for 5 minutes over a gentle heat, stirring occasionally. Remove the bay leaves, add nutmeg and season to taste.
For the pesto:
Add the Lurpak® Slightly Salted Butter, basil, pine nuts, parmesan and garlic to a food processor and whizz to make a thick puree, and season.
Butter a 2 litre ovenproof dish and start with a layer of lasagne sheets, then a thin layer of white sauce and pesto, then sweet potatoes and Parmesan cheese, reserving a handful for the top and season with freshly ground black pepper. Repeat the layers until the sweet potato is used and finish with the white sauce.
Dot the ricotta over the top, scatter with the reserved Parmesan and bake in the oven for 1 hour 30 minutes until golden and bubbling, cover with foil if browning too much. Serve with a green salad.
Tip: This pesto is delicious just tossed through cooked pasta and finished with Parmesan and ricotta cheese.