Let the puff pastry rest for about 10 minutes at room temperature. In the meantime peel onions and cut them lengthwise into thick slices. Fry bacon in a pan without fat until crispy. Take out and drain on kitchen paper.
Heat the oil in the bacon fat, fry the onions for about 4 minutes, turning them over, but do not let them get too brown. Season with salt and pepper.
Preheat oven (electric cooker: 225 °C/circulating air: 200 °C/gas: see manufacturer). Line baking tray with baking paper. Mix sour cream and mustard. Season to taste with salt, pepper and 1⁄2-1 tsp. caraway seeds.
Unroll the puff pastry, remove from the paper. Cut the dough sheet in half lengthwise and divide it into thirds crosswise, so that 6 pieces of the same size are formed. Place on the tray. Sprinkle each piece of dough with 1 tsp. breadcrumbs. Spread the sour cream evenly on top, leaving about 2 1⁄2 cm free all around.
Spread the onions on the cream.
Overfold the edge of the puff pastry slightly towards the middle. Whisk the egg yolk and 1 tbsp. water. Spread the edge of the pastry with it. Bake in a hot oven for 15-18 minutes on the lower shelf. Take out and put 1 slice of bacon on each.
Arrange the tartes on salad leaves if necessary.