Mix flour, yeast, milk, margarine, and salt into a dough. Let the pre-dough rise for 25 minutes, then let the finished dough rise for 45 minutes.
While dough rises, cut bacon into small cubes and cook in a pan over medium heat for 3-5 minutes. Add finely chopped onions and cook for 2 minutes more. Season with salt, pepper, and finely chopped parsley to taste.
Roll out the risen dough and cut into 8 cm diameter circles or squares. Place filling on half of each piece, brush the edge with whisked egg white, fold the other half over to seal, and press edges firmly together.
Place assembled rolls on a greased baking sheet and let rise again until roughly doubled in size.
Brush each roll with egg yolk mixed with a little milk. Bake on the middle rack at 210°C (410°F) for 25 minutes until golden.