Polish stuffed dumplings with marinated mushrooms and potatoes, with onion and marjoram

Ingredients:
Dough:
● 300 g all-purpose flour
● 1 egg (optional)
● 150 ml warm water
● 1 tablespoon oil
● a pinch of salt
Filling:
● 1 cup  marinated mushrooms with carrots and onions OLE! (drained, very finely chopped)
● 2-3 boiled potatoes (puree)
● 1 small fried onion
● ½ teaspoon marjoram
● salt and pepper to taste
To serve:
● butter with parsley and garlic
● thick natural yogurt/sour cream