Peel and finely dice the shallots. Melt 1 teaspoon butter in a saucepan. Sauté shallots in it while stirring until translucent. Deglaze with vermouth, bring to the boil and reduce at low heat for 2-3 minutes.
Add crème fraiche and stir in. Season to taste with salt, pepper and nutmeg, put aside.
Clean and wash the leek and cut it into fine strips of about 3 cm. Melt 1 teaspoon of butter in a pan and fry the leek for 3-4 minutes until colourless. Season with salt and nutmeg. Heat oil in another coated pan.
Rinse scallops cold, dab dry and fry them in a hot pan for about 1 minute on each side, season with salt and pepper. Remove.
Take the puff pastry out of the refrigerator 5-10 minutes before use. Divide the leek strips into 4 ovenproof moulds (each approx. 9 cm Ø). Pour the sauce over them and place 2 scallops in each mould.
Roll out the puff pastry. Cut out 4 circles (each approx. 11 cm Ø). Use the rest of the dough for other purposes. Whisk egg yolk with 1 tbsp. water. Brush the edges of the moulds. Spread puff pastry circles on top.
Press the edges firmly, spread the dough with egg yolk as well. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see manufacturer) for approx. 12 minutes until golden brown.
Remove from the oven and serve immediately. Garnish with thyme if desired.