Place the puff pastry slices next to each other and defrost. Peel onions, slice or cut them into thin rings. Dice bacon finely.
Heat the fat in a frying pan. Fry the bacon in it until crispy and take it out. Steam onions in the frying fat for approx. 10 minutes, turning them over. Fold in bacon again. Season with pepper and a little salt. Let cool down.
Grease 4 small quiche moulds (approx. 15 cm Ø) or 1 large one (30 cm Ø). Roll out the dough slices slightly larger on a little flour. Place them individually in the moulds and press the edges together. Cut off any protruding edges.
Prick several times with a fork.
First spread breadcrumbs, then onion mixture into the moulds. Whisk milk, cream and eggs. Season with salt, pepper and nutmeg. Pour over the onions. Bake in a hot oven on the lowest level (electric oven: 200 °C/circulating air: 175 °C/gas: level 3) for approx. 30 minutes.