Warm the milk until lukewarm, then stir in the sugar and yeast. Let it stand for 10 minutes, until frothy.
Sift the flour into a large bowl, add the salt, then pour in the yeast mixture and the egg. Start to knead the dough.
Knead the dough until smooth, then cover with a clean kitchen towel and let rise in a warm place for about 1 hour.
Roll out the risen dough into a rectangle and place thin slices of 200g cold butter on it. Fold one half of the dough over the other, then roll out again and fold. Repeat this folding process three times, allowing 15-minute resting periods in between.
Cut the ham into small pieces and mix with the grated cheese and mustard to create a flavorful filling.
After the final rolling, cut the dough into triangles. Place a small amount of ham and cheese filling in the center of each triangle, then roll up into a croissant shape.
Place the croissants on a baking sheet lined with parchment paper and let them rise for another 30 minutes.
Preheat the oven to 350°F (180°C). Brush the croissants with a little beaten egg and bake for 18-20 minutes, or until golden brown.
When the croissants are baked, let them cool slightly before serving fresh.
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