Clean and wash the spinach leaves. Put the spinach dripping wet in a large saucepan and let it collapse over high heat. Drain well on a sieve.
Heat 1 tablespoon of olive oil in a large pan, add the spinach, press in the clove of garlic, season with salt and pepper, mix in the butter and let it melt over a mild heat. Take the pan off the stove.
Preheat the oven to 200 ° C top / bottom heat.
For the sauce, melt the butter in a saucepan, stir in the flour and sweat. Gradually pour in the milk, stirring constantly, and simmer for about 5 minutes. Remove from heat, stir in 25 g parmesan, season with lemon juice, salt, pepper and nutmeg.
Spread the spinach in an ovenproof dish, pour the sauce over it and sprinkle with the remaining 25 g of parmesan. Baked in the oven for about 5 minutes.