Finely grate white cabbage. Peel and dice onion. Heat sunflower oil in a frying pan, add onions and cabbage and let steam until cabbage is slightly browned and softened.
Boil eggs until cooked through, let cool, and dice into cubes.
In a bowl mix cabbage and eggs, and season lightly with salt and pepper. Set aside and let cool.
For the dough, melt the butter and let cool slightly. Add milk and yeast to the melted butter, and stir to combine. Crack eggs into a large bowl and whisk together with sugar and salt. Add the butter-milk-yeast-mixture. Gradually add flour and knead the dough (use a stand mixer with a dough hook attached, or use your hands) until it comes together and no longer sticks to the sides of the bowl. Dust with flour and let rest at room temperature until doubled in size.
Preheat the oven to 200°C/390°F. Divide the dough into 6 equal pieces (2 per pirogi). Gently dust your work surface with flour, and roll out the dough to a 0.25 in/0.5cm thickness and place on a greased, or parchment-paper-lined baking sheet. Add filling, leaving some space at the edges.Roll the bottom edges up tightly to meet the pastry lid to create a seal. Pierce the top with a fork. Beat an egg, then brush the dough with it.
Transfer pierogis to the oven and let bake for approx. 15 – 20 min. until golden brown. Remove and let cool slightly. Serve with cold milk.