In a small saucepan, cover the mushrooms with hot water and bring to the boil. Cook for 20 minutes until tender then drain and chop.
Melt the butter in a saucepan then add the onions and cook for a few moments. Stir in mushrooms and cook until the onion is soft. Season with salt and pepper to taste. If the mixture seems to loose, add some breadcrumbs. Set aside.
For the dough:
Combine flour with egg yolk and hot milk, quickly stir with a fork until well incorporated. The dough should be smooth and soft. If it is too stiff, pour in some more milk.
On a lightly floured surface, roll out dough and cut into 4cm squares. Place a teaspoon filling in the centre of each square. Fold the dough over the filling to form a triangle, then pinch the edges to seal. Bring the opposite endings and pinch together.
Bring a large saucepan of salted water to the boil. Place dumplings in batches and cook for 10 minutes until they float on the surface. Use a slotted spoon to take them out.