- 1 tablespoon Kosher salt
- 5 medium Yukon Gold potatoes (about 1¾ pounds)
- 6 tablespoons unsalted butter, divided
- 2 leeks, white and pale-green parts only, sliced in half lengthwise, thinly sliced crosswise
- 2 garlic cloves, thinly sliced
- 1 ¼ cups milk
- ½ cup heavy cream
- Freshly ground black pepper
- 1 scallion, thinly sliced
- Cover potatoes with water in a small pot; season with salt. Bring to a boil over medium-high heat, then reduce heat and simmer until potatoes are fork tender. Drain, let cool slightly, and peel.
- Meanwhile, melt 4 Tbsp. of the butter amount in a large saucepan over medium heat. Add leeks and cook, stirring frequently, until very soft, 8–10 minutes. Add garlic and cook, stirring frequently, until garlic is fragrant and leeks are just beginning to brown around the edges, about 3 minutes longer. Add milk and cream and bring to a simmer.
- Add boiled potatoes then coarsely mash with a potato masher. Season with more salt if needed and pepper.
- Transfer colcannon to a large serving bowl. Top with remaining 2 Tbsp. butter and sprinkle with scallion.