Directions

1. Position racks in middle and lower thirds of oven; preheat to 450°F. Line a large rimmed baking sheet with parchment paper.
2. Place potatoes in a large pot and cover with water; bring to a boil. Reduce heat to medium-high and simmer, covered, until tender when pierced with a fork, 10 to 15 minutes. Drain the potatoes well and pat dry with a clean kitchen towel; transfer to a large bowl.
3. Meanwhile, heat 2 tablespoons oil and butter in a large skillet over medium heat. Add onions and 1/4 teaspoon salt; cook, stirring occasionally, until soft and translucent, 5 to 7 minutes. Reduce heat to medium-low and cook, stirring often, until the onions are golden brown and jammy, 20 to 35 minutes. Add water as needed, 1 tablespoon at a time, stirring with a wooden spoon to scrape and stir in any browned bits sticking to bottom of the skillet. Remove from heat and transfer the onions to a small bowl.
4. Drizzle 1 tablespoon oil over the potatoes. Sprinkle with 1/4 teaspoon each salt and pepper, and toss to coat. Spread the potatoes evenly on the prepared baking sheet. Using a sturdy glass jar, gently press down on the potatoes to smash to 1/2-inch thickness. The potatoes should stay intact.
5. Drizzle the potatoes with the remaining 1 tablespoon oil. Bake, flipping once halfway, until golden brown and crispy, 20 to 25 minutes. Remove from oven and turn broiler to high.
6. Top the potatoes evenly with the caramelized onions and cheese, about 1/2 teaspoon onions and 1 1/2 teaspoons cheese per potato. Broil on the upper rack until the cheese is melted and bubbly, about 2 minutes. Immediately sprinkle with the remaining 1/4 teaspoon salt. Top with chives and the remaining 1/4 teaspoon pepper. Transfer to a platter and serve warm.