Combine flour and salt in bowl of a stand mixer fitted with a dough hook and mix at low. Add butter and egg and then 225 ml (1 scant cup) of ice water. Mix until smooth—5 to 6 minutes. Remove dough, roll into a ball and cover with plastic wrap. Place in fridge for at least an hour. Meanwhile, preheat oven to 200°C (400°F). Transfer potatoes to centre rack and bake until tender—about 1 hour and 10 minutes. Remove to countertop to cool slightly, then split lengthwise. Place a wire rack on top of a bowl, arrange potato halves on top, flesh side down, and push the flesh through. Transfer about 500 ml (2 cups) of the crumbled potato to a mixing bowl, add the cheeses, salt and pepper, and combine until smooth.

Let dough rest on countertop for 20 minutes. Then pass it through a pasta maker to form thin sheets. Cut these into 8-cm (3-inch) squares and arrange on a work surface. Place 1 tbsp of filling at centre of each square, then fold dough corner to corner to form triangles. (If the dough is dry, first brush inside edges with water.) Then pinch to seal.

Bring a large pot of salted water to a rolling boil. Add perogies and cook until they float. Remove with a slotted spoon to a lightly oiled tray. Heat a large skillet on medium and heat remaining oil until it shimmers. Add perogies and then butter. Once bronzed, flip, then distribute among warm plates—about 8 per serving. Garnish perogies with warm onions and lardons, sprinkle with dill and paprika, and finish with a few dollops of sour cream.