The simplest way to prepare the pierogi dough is to mix 2 cups plain flour, 1/2 teaspoon salt, 1 large free-range egg, 1/2 cup sour cream and 1/4 cup butter together in a food processor with a dough blade. Pulse until it starts to form clumps, but be careful not to overbeat. Wrap the dough in plastic and refrigerate for 20-30 minutes or overnight.
Meanwhile, boil 5 medium potatoes until they are tender. Mash and allow to cool. Chop 1 onion finely and saute in 20 g butter. Cool. Mix together the potatoes, onions, 75 g mature cheddar cheese, 1 tbsp fresh parsley and season to taste with sea salt & freshly ground black pepper.
Roll the pierogi dough on a floured board or countertop until 1/8" thick. Cut circles of dough (2" for small pierogies and 3-3 1/2" for large pierogies, I like mine large) with a cookie cutter. Place a small ball of filling, around a tablespoon, on each dough round and fold the dough over, forming a semi-circle. The dough is quite elastic so you can stretch it around. A little bit of practise and this becomes very easy. Moisten the edges with water and seal. Press the edges together with the tines of a fork.
Boil the perogies a few at a time in a large pot of water. They are done when they float to the top (about 8-10 minutes). Remove from the water with a slotted spoon and allow them to drain on a wire rack.
Saute 2 large onions, sliced, and 6 rashers smoked back bacon in 1/4 cup butterin a large pan until onions are soft and the bacon is cooked. Remove from the pan and keep warm. Pan fry the pierogies in more butter until lightly crispy.