Ingredients For frying 2 tablespoon unsalted butter 2 tablespoons vegetable oil, divided filling 1 medium potato, skin on 1/4 cup diced onion 2 tablespoons ricotta cheese dough 1 1/2 cups all-purpose flour, divided 1 large egg 1/4 teaspoon kosher salt 3 tablespoons water, divided 1 teaspoon table salt for boiling water bath   Filling   Cover potato, skin on, with water in a 2-quart saucepan. Bring to a boil. Boil for 1 minute. Reduce heat to medium, cover and cook until fork tender but not mushy about (10 to 15 minutes). Remove potato, cool until easy to handle then peel. Place peeled potato in a small bowl and mash with a fork.   While potato cooks, sauté spinach and onions in 1 tablespoon vegetable oil in a small fry pan over medium heat for about 2 minutes until spinach wilts and onions turn translucent.   Mix together potato, spinach mixture and ricotta cheese. Set aside.   Dough   Place 1 cup flour on cutting board. Make a well in the center. Add salt, egg and 1 tablespoon water at a time. Knead into soft dough. Adjust water and flour as needed from reserve. Form into a ball. Cover with damp towel and rest 10 minutes.   Roll out dough to 1/4-inch thickness. Cut into nine 3-inch rounds. Set aside.   Add 1 rounded teaspoon of filling in the center of each circle. Wet the edges with a wet fingertip. Fold over filling and gently seal edges with a fork. Bring 2 quarts water and table salt to a boil in a large pot or dutch oven over medium-high heat. Gently drop in pierogies, one at a time with a slotted spoon. Stir. When water returns to a boil, reduce heat and cook about 6 minutes total. Pierogies will float after about 3 minutes. Continue cooking another 3 minutes to cook dough more thoroughly. Move cooked pierogies to a plate. Fry or refrigerate, covered, for later use. Pierogies can be eaten as is, deep fried or pan fried. To pan fry: Fry pierogies in butter and oil in a medium-sized skillet over medium heat until crisp and browned. Serve with caramelized onions and sour cream if desired.