Drain the potatoes and return to the pot over low heat. Allow the potatoes to steam in the hot pot, tossing occasionally to drive away any excess moisture, about 2 minutes. Add the sour cream, ½ cup milk, onion powder and lots of pepper. Use a potato masher to roughly break up the potatoes, then switch to a spatula to stir well (you want some lumps). Add more milk if needed. Taste and add salt as needed. Add the scallions, reserving a small handful for garnish, and stir well to combine. Taste and add more salt as needed. Top with reserved scallions and serve right away.
