Defrost the spinach in a sieve. For the herb béchamel, meanwhile melt 75 g butter in a large pot. Stir in the flour and sweat. Gradually stir in the milk. Simmer for about 2 minutes while stirring. Season with salt, pepper and nutmeg. Stir in crème fraîche.
Squeeze out the thawed spinach and place in an ovenproof dish (approx. 30 cm Ø). Rinse salmon, dab dry and cut into approx. 3 cm wide slices. Place them side by side on the spinach. Pour over the béchamel.
Wash the potatoes thoroughly and slice them into 2 mm thin slices. Place them in scales on the salmon. Season with salt.
Preheat oven (electric cooker: 200 °C/circulating air: 180 °C/gas: see manufacturer). For the Mediterranean butter, peel garlic and chop finely. Wash the thyme and dab dry. Heat 2 tablespoons of butter in a small pot. Fry the garlic and thyme lightly for about 2 minutes. Brush the potato slices with the flavoured butter.
Bake the casserole in a hot oven for about 45 minutes until golden brown, covering with aluminium foil after 25 minutes if necessary.