Heat the oil in a large pan and fry the onions for 10 mins, until starting to turn golden. Stir in the aubergines, then fry 5 mins more to soften them.
Pour in the passata and wine, then stir in the capers, sugar, 1 tsp salt and some black pepper. Cover and cook for 20 mins.
Meanwhile, spoon 250g mascarpone into a bowl with the milk, pesto and eggs, and beat until smooth.
Halve each salmon fillet to make 2 long pieces. Spread 1 tbsp of the remaining mascarpone over each pasta sheet, add a piece of salmon, then season
and roll up.
Wash and wilt the spinach in a large pan (you don’t need to add oil), then squeeze as much liquid as you can from it – this is important as otherwise the pasta bake will be too wet.
Spread the spinach on the base of a large lasagne dish or roasting tin, measuring approx 27 x 35cm. Arrange the salmon and lasagne parcels on the spinach, then pour over the tomato and aubergine sauce. Now carefully pour over the pesto and mascarpone layer. If freezing, cover the surface of the lasagne with cling film, then overwrap the dish with cling film and foil. To defrost, thaw overnight in the fridge.
To cook and serve, heat oven to 190C/170C fan/gas 5, scatter the Parmesan over the mascarpone and bake for 40 mins until bubbling. Allow to settle in the dish for 5-10 mins before serving scattered with basil leaves.