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Heat oven to 180C/160C fan/gas 4. Place the pesto, half the mascarpone and 250ml water (or vegetable stock if you have some) in a pan. Heat and mix until smooth and bubbling. Add the spinach and peas and cook for a few more mins until the spinach has wilted and the peas thawed. Add the herbs and season.
Place a third of the pesto mixture into a baking dish roughly 18 x 25cm. Top with 4 lasagne sheets, then repeat with 2 more layers of sauce and lasagne sheets, finishing with a layer of pasta. Mix enough milk into the remaining mascarpone to make a white sauce consistency, season, then pour over the top. Sprinkle with the Parmesan and pine nuts. Bake for 35-40 mins until golden brown on top and bubbling around the edges. Scatter over the reserved basil leaves and serve with a green salad, if you like.