Using a fine mesh sieve or cheesecloth, set the ricotta cheese to drain over a medium bowl.
In a large mixing bowl, combine the flour and salt.
Create a well in the center of the flour mixture. Add the eggs, sour cream, and melted butter into the well.
Using a fork or your hands, mix the ingredients together. Gradually add the warm water, until the dough starts to come together.
Transfer the dough to a lightly floured surface and knead it for about 5-7 minutes until it becomes smooth and elastic. Add more water or flour as needed to reach the right consistency.
Wrap the dough in plastic wrap or cover it with a clean kitchen towel. Let it rest for at least 30 minutes at room temperature.
Transfer the drained ricotta to a large mixing bowl. Add finely chopped fresh dill, chopped green onions, salt, pepper, nutmeg, and lightly whisked egg.
Use a spoon or spatula to mix everything until it is well combined. Make sure the egg is evenly distributed throughout the mixture.
If the filling seems too soft or if you want to make it easier to handle, you can chill it in the refrigerator for about 30 minutes. You can also add cornstarch or flour to absorb the extra moisture,
On a floured surface, roll out the dough to about 1/8-inch thickness. Cut out circles of dough using a round cutter (about 3 inches in diameter).
Place a small spoonful (1.5 tsp) of your filling in the center of each dough circle. Fold the dough over to create a half-moon shape and press the edges together to seal. Crimp the edges with a fork for extra security.
While you are assembling the pierogi, bring a large pot of salted water to a boil.
Add the pierogis in batches, making sure not to overcrowd the pot. Boil them until they float to the surface, which takes about 2-3 minutes. Remove them with a slotted spoon and drain.
Notes
Drain ricotta - make sure to drain the ricotta cheese well to avoid a runny filling. You can use a fine-mesh sieve or cheesecloth for this.Use room temperature ingredients - eggs and sour cream should be at room temperature to mix more easily with the flour.Rest the dough - let the dough rest for at least 30 minutes. This relaxes the gluten and makes the dough easier to roll out and handle.Cook gently - add the pierogies to boiling water in batches and cook them gently. Once they float to the surface, cook for an additional 2-3 minutes.