For the chard gratin, first rinse the chard and cut the stems into small pieces. If necessary, peel off the outermost tough skin from the stems and cut the stem base into narrow strips. Chop the leafy parts and set aside separately from the stems.
Remove the peel from the onion, dice and sauté in the hot fat until light. Squeeze the garlic clove, add it and add the chard stems. Season the vegetables lightly with salt, put the lid on and steam the vegetables for about 5 minutes, preferably without or with little liquid added. Now add the chopped leaves and steam the vegetables with the lid on for another 10 minutes. Season the chard vegetables again with salt, pepper and a pinch of nutmeg.
Steam the potatoes in a little water for 20-30 minutes. Then peel the potatoes and cut them into thin slices. Grease a shallow baking dish and layer the potato slices on the bottom like a roof tile. Then season the potatoes lightly with salt and pepper and dust with paprika powder. Spread the chard vegetables evenly over the potatoes.
Mix sour cream, crème fraîche, eggs and spices to make a glaze and empty it over the top layer. Spread the grated cheese evenly over the chard gratin and bake it at 200 degrees (hot air 180 degrees) for about 20 minutes.