Take the puff pastry out of the fridge and let it rest for about 10 minutes. Cut the ham into cubes. Peel and finely dice onion. Wash parsley, shake dry, pluck leaves from the stalks and chop.
Heat butter. Fry diced onions for about 5 minutes while stirring until transparent. Add mustard, cream, ham, almonds and parsley and stir in. Season to taste with salt and pepper. Grate cheese. Roll out the puff pastry and cut in half lengthwise.
Cut each strip into 3 rectangles of the same size and halve them diagonally. Spread the filling over the triangles and sprinkle with cheese. Roll up the puff pastry from the long side to the top and place on a baking tray lined with baking paper.
Whisk the egg yolk and milk and spread the croissants with it. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 15-20 minutes. Take the finished croissants out of the oven and let them cool down.