Chop the onion. Heat up the butter, add onion and cook a few minutes.
Place in a pot. Using a potato ricer or potato masher. Add cheese, salt and pepper. Mix together.

Pour the flour on a counter, add salt and mix. Make a little whole in the middle. Add yolk and oil. Mix together. Add a little water at the time. Continue kneading with your hand. Divide into two parts. Cover it with the kitchen towel and let it rest for 15 mins.

Make a balls.
Roll the dough. Cut the circles.
Slightly stretch the sides of the dough. Place the ball of the filling in the middle of the circle. Seal it together.

Bring a pot of lightly salted water to a boil. Add dumplings and cook for 2 minutes or until float to the top. Remove with a slotted spoon.
Video step by step https://youtu.be/pV7uKEHotaM