DOUGH: In bowl, combine flour with salt. Beat together egg, water and oil; stir into flour mixture to make soft but not sticky dough that holds together in a ball. If necessary, add 1 Tbsp more water at a time, being careful not to make dough sticky. Turn out dough onto lightly floured surface; knead about 10 times or till smooth. Halve dough, cover with plastic wrap or damp cloth. Let rest for 20 minutes. FILLING: Meanwhile in skillet, heat butter over medium heat, cook onion for 3 to 5 minutes or till tender. Transfer to a bowl, add mix in potatoes, cheese, salt and pepper. Working with one portion of the dough at a time, and keeping the remaining dough covered, roll out on a lightly floured surface to 1/16" thickness. Using 3 inch round cutter, cut dough into rounds. Place 1 tsp filling on each round. Lightly moisten edge of one half of dough with water, pinch edges together to seal and crimp attractively. Place on cloth; cover with damp cloth to prevent drying out. Repeat with remaining portion of dough. In large pot of boiling salted water, cook perogies in batches, for 1 1/2 hr to 2 minutes or till they float to top, stirring gently to prevent perogies from sticking together or to bottom of pan. With slotted spoon remove to colander to drain. In large heavy skillet, melt butter over medium heat, cook onion for about 5 minutes till golden. Add perogies and toss to coat and warm through. Serve with sour cream. MAKES ABOUT 30 PEROGIES. Variations: Cottage Cheese: Combine 1 cup press cottage cheese, 1 beaten egg, 1/2 tsp salt, 1/4 tsp pepper and 1 Tbsp chopped green onion. Mushroom: In skillet, melt 2 Tbsp butter over medium heat, cook 3 cups chopped mushrooms and 1/3 cup finely chopped onion for 7 to 9 minutes or until moisture has evaporated. Remove from heat; stir in 1 egg yolk and 1 Tbsp chopped fresh dill. Season with salt and pepper to taste.