In a large Dutch oven over medium, heat the oil until shimmering. Add the onion and ¼ teaspoon salt, then cook, stirring occasionally, until softened and well browned, 22 to 25 minutes. Remove the pot from the heat. Transfer the onion to a small bowl and set aside; reserve the pot.

In a colander under cold running water, rinse the potatoes. Drain well, then add to the pot. Stir in ¾ cup water and ½ teaspoon salt, then distribute the potatoes in an even layer.

Cover and bring to a boil over medium-­high. Reduce to medium and cook at a simmer, stirring occasionally, until the slices almost fall apart when poked with a skewer, 18 to 20 minutes.

If there is water remaining in the pot, increase to medium-high and cook, uncovered and stirring often, until no moisture remains. Reduce to low, add the butter and cook, stirring and mashing the potatoes with a spoon, until the butter is melted and incorporated, about 1 minute.

Stir in the onion, paprika, and ¼ teaspoon each salt and pepper. Stir in 1 tablespoon of the chives, then taste and season with salt and pepper. Transfer to a serving dish. Sprinkle with additional paprika and the remaining 1 tablespoon chives.