Cut ham, mozzarella and peppers into small pieces. Coarsely grate Gouda. Peel and chop the onion.
Mix all ingredients, stir in the egg and season with salt and pepper.
Roll out the puff pastry and spread the filling.
Roll up the strudel and brush with melted butter. Bake in the preheated oven at 180 °C for about 25 minutes.