Boil 6 large potatoes for 15 minutes. Peel and slice them.
Dice the onion. Cut the leek into rings. Chop the Swiss chard finely. Snip the chives and dill with scissors. Mix the sour cream with the crème fraîche and some pepper.
Butter a baking dish. Layer half the sliced potatoes on the bottom. Scatter half the diced onion and leek rings over them. Sprinkle half the chives and dill over the vegetables. Arrange half the herring fillets, cut into pieces, on top. Pour half the sour cream mixture over this layer.
Repeat with the remaining potatoes, onion, leek, chives, dill, Swiss chard, and herring fillets. Pour the remaining sour cream mixture over the top. Sprinkle the toasted bread cubes over everything and press lightly.
Bake at 200°C (392°F) on the lowest rack of the oven for 40 minutes.