For the dough: Add to a mixer the flour, salt, sour cream, egg and 1 1/4 cups water. Mix these ingredients until the dough comes together and is soft. Divide the dough in half and set on a floured surface. Roll each half of the dough into a thin circle, about 1/8 inch in thickness. Cut the dough into 3-inch rounds using a biscuit cutter.
For the blueberry filling: Pour the blueberries into a medium-size bowl and mix them with the flour and sugar. Set this aside.
For the farmer's cheese filling: In a medium-size bowl, mix the cream cheese, farmer's cheese, egg, salt and pepper together. Set this aside.
For the salt pork filling: Add the butter to a saute pan over medium heat. Add in the salt pork and onions and cook for 10 minutes. When the salt pork is browned, add in the sauerkraut and garlic and cook for 5 to 10 more minutes.
To fill the pierogi: Place a small spoonful of the filling into the center of each 3-inch round of dough. Moisten the edges with water, fold over and press the dough edges together with a fork to seal them. Repeat this process with the remaining dough and all fillings.
Bring a large pot of lightly salted water to a boil. Add the pierogi and cook them until they float to the top, 3 to 5 minutes. Remove the pierogi with a slotted spoon to a bowl of cold water, and then drain them. Top the pierogi with melted butter or sour cream and serve warm.