Defrost and drain the spinach in a sieve.
Press the garlic, finely chop the shallots.
Heat the oil in a non-stick frying pan. Sauté the garlic, shallots and thyme for approx. 3 mins., add the spinach and continue to cook for approx. 5 mins., season and set aside.
Heat the butter in a pan. Add the flour and cook, stirring with a whisk; the flour must not take on any colour.
Remove the pan from the heat. Pour in the milk, bring to the boil while stirring, reduce the heat, season, simmer for approx. 10 mins., stirring occasionally until the sauce is thick and creamy. Stir in the mozzarella and cheese.
Layer the spinach, lasagne sheets and béchamel sauce in the prepared dish. Cover with a final layer of béchamel sauce, scatter mozzarella and cheese over the top.
Bake for approx. 40 mins. in the centre of an oven preheated to 200°C. Remove from the oven and leave to stand for approx. 5 mins.