Finely chop the braised sauerkraut with mushrooms.
Toast the ⅓ cup walnuts in a dry skillet for 3-4 minutes, until fragrant; cool and finely chop.
Slice 2 large yellow onions for the caramelized onion–prune sauce.
Measure the spices and liquids, and set out a small bowl of water for sealing pierogi.
In a large bowl, whisk 3 cups all-purpose flour with 1 tsp fine salt
Make a well and add 1 large egg, 1 cup warm water, and 22 tablespoon neutral oilor.
Mix until shaggy, then knead for 6 - 8 minutes until smooth and elastic.
Shape into a ball, lightly coat with oil, cover, and let rest for 20–30 minutes.
Heat2 tbsp butter and 1 tbsp neutral oil in a skillet over medium-low heat.
Add onions and a pinch of salt; cook slowly for 25–30 minutes, stirring occasionally, until golden and sweet.
Stir in chopped 6 - 8 dried prunes and cook for 2–3 minutes until softened.
Add ¼ cup hot water or stock and simmer for 5–7 minutes until spoonable.
Stir in 2 tsp balsamic vinegar gradually, then finish with black pepper. Keep warm until serving.
In a medium pan, combine 2½ cups braised sauerkraut with mushrooms, 1½ teaspoons dried marjoram, ¼ teaspoon black pepper.
Fold in ⅓ cup walnuts (toasted).
Cook for 5–7 minutes over medium heat to remove any excess moisture.
Remove from heat and let cool completely before filling the pierogi.
Roll the rested dough to ~1/8" thickness on a lightly floured surface.
Cut into 3-inch rounds.
Place ~1 tbsp of cooled filling in the center of each round.
Brush the edges with water, fold in half, and crimp the edges securely.
Place finished pierogi on a floured tray, not touching, until ready to cook.
Bring a large pot of salted water to a gentle boil.
Drop the pierogi in batches. When they float, cook 1–2 more minutes.
Remove with a slotted spoon and set aside.
Melt 6 tablespoons unsalted butter in a skillet over medium heat, swirling occasionally.
When it turns nutty and amber, remove from heat and add torn 10 sage leaves; they will crisp.
Stir in ¼ cup walnuts to warm through.
Spoon over hot pierogi or toss gently in the skillet to coat. Finish with flaky salt
Notes
Chop the filling finely - smaller pieces seal better, cook more evenly, and give you that smooth, cohesive bite.Cool the filling completely - warm filling softens the dough and makes sealing harder. Don’t overfill - it’s tempting, but one generous tablespoon is plenty. Overfilled pierogi burst in the water.