Place a medium pot of water over high heat and bring to a boil. Place a bowl of ice water near the pot. Add the spinach to the boiling water and cook just until vivid green, about 30 seconds. Use a slotted spoon to transfer the spinach to the ice water. Once chilled, drain, and squeeze using a cheesecloth or towel to remove any excess moisture. Roughly chop.
Place a large, nonstick skillet over medium low heat and drizzle with vegetable oil.
In a bowl, whisk together the eggs with the milk and 1/4 teaspoon of the salt. Add to the heated skillet and cook, stirring occasionally, until just fluffy but not completely set, about 4 minutes. Remove from heat and transfer to a medium bowl. Gently fold in the sesame oil, oyster sauce, remaining salt, and chopped spinach.
Bring a large pot of water to a boil.
On a lightly floured surface, knead the dough briefly and divide the rested dough into 24 equal pieces. Roll a piece into a thin circle 3-4 inches wide. Repeat with remaining wrappers and place until a lightly moistened towel to keep them from drying out.
Place about a tablespoon of the prepared filling in the center of a wrapper. Fold the dumpling into the round yuan bao shape, squeezing out any air bubbles as you seal the dough. Place on a parchment lined baking sheet. Repeat with remaining wrappers and filling.
Transfer 6 dumplings to the boiling water, stirring briefly to keep them from sticking to the bottom. Boil over high heat for 2 minutes, then reduce heat to medium high and cook for an additional minute. Reduce heat to medium and cook for two more minutes, until risen to the top and puffy, before removing with a slotted spoon. Bring the water back to a boil over high heat and repeat with remaining dumplings. Serve immediately.