Bring a saucepan three-fourths full of water to a boil over high heat. Add the onions and blanch for 2 minutes to loosen the papery skins. Drain and rinse under running cold water. Slip off the skins, then trim only the tip of each root, leaving the base intact. This helps the onions hold their shape during cooking.
In a sauté pan just large enough to hold the onions in a single layer, melt the butter over medium-high heat. When it foams, add the onions and cook, stirring often, until lightly golden, about 12 minutes. Add
Transfer the onions to a bowl and let cool. Cover and refrigerate for up to 1 week before using. Bring to room temperature before serving.