Place puff pastry slices next to each other and defrost at room temperature. Peel onions and cut into thin strips. Dice bacon. Melt the fat in a saucepan, add the onions and bacon and steam over a low to medium heat for about 15 minutes, stirring occasionally.
Season with salt and pepper. Whisk milk, cream, eggs and egg yolks in a bowl. Season with salt, pepper and nutmeg. Puff pastry slice
Grease 4 tartlet cups (15 cm Ø), line with the dough, press the edges. Cut off protruding corners. Spread the onions and egg milk in the tartlets. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 30-40 minutes.
Serve hot.