Chop the onion and add to a saucepan with the olive oil. Saute over a medium heat for a couple of minutes, then add the spinach (you'll need to do this bit by bit!) and stir until it has all wilted. Stir in the nutmeg, and season. Leave to sit in the saucepan, off the heat.
Put the creme fraiche, ricotta and egg into a large bowl and beat until well combined. Seson with salt and plenty of black pepper.
Half the cherry tomatoes.
Drain any excess liquid from the spinach, and you can start layering your lasagne.
In a large ovenproof dish, layer your ingredients. Half the spinach, half the tomato, one third of the lasagne sheets, half the ricotta mix, one third of the lasagne sheets, the remaining spinach, the remaining tomato, the remaining lasagne sheets, the remaining ricotta mix. Make sure you top with the ricotta!
Top with grated parmigiano reggiano and bake in the oven at 180 degrees celsius for 30 minutes, or until golden brown.