The evening before, mix the flour, oat flakes, ground hazelnuts, salt and dry yeast in a mixing bowl. Then use the dough hook of the hand mixer to stir in the honey, lukewarm milk, softened butter and eggs, so that a soft, slightly sticky dough is created that just loosens from the edge of the bowl. Cover it with a damp cloth and let it rise in a warm place for about 45 minutes.
Dust the worktop with flour and roll out the dough into a circle about 40 cm in diameter. Cut 8 pieces of cake out of the circle and roll up each one from the edge of the circle towards the tip, so that croissants are formed. Brush the croissants with milk, sprinkle with the nuts and place on a baking sheet lined with baking paper. Cover the tray again with the damp cloth and store in the refrigerator overnight.
The next morning, take the tray out of the refrigerator, let rise in a warm place for about 30 minutes and then bake in a preheated oven at 200 ° C top / bottom heat for 20-30 minutes until golden yellow.
If there are croissants left over, you can reheat them briefly in the oven the next day.