First, make the white sauce. Melt the butter in a saucepan, then add the plain flour. Stir continuously until a paste forms and continue cooking for 2 mins. Add the milk to the roux gradually, stirring as you go, until you get a smooth sauce. Keep cooking for 5-10 mins more, stirring continuously, until the sauce has thickened. Leave to cool.
To make the spinach balls, cook the spinach in the butter until it is warmed through, then stir in a grating of nutmeg and the garlic. Fry for a minute, then stir in the breadcrumbs and cheese. Tip into a dish and stir in the egg to form a stiff paste. Wet your hands and roll about 18 spoonfuls of the mixture into balls. Put them on a baking tray and chill for 30 mins.
Heat oven to 190C/170C fan/gas 5. Fry the spinach balls in batches in the oil until lightly browned. Spoon some white sauce into the base of a medium baking dish and tip in six spinach balls. Drizzle over 2 tsp pesto, then place half the lasagne sheets on top, overlapping, add more white sauce, six more spinach balls, then a little more pesto. Place the remaining lasagne sheets on top, cover with white sauce, then dot over the last six spinach balls. Drizzle over more pesto and scatter over the remaining cheese. Bake for 30 mins or until the lasagne sheets are cooked through. Scatter over some basil leaves, if you like.