Position racks in upper and lower thirds of oven; preheat to 450°F.
Combine water and salt in a large pot; bring to a boil over high heat.
Add potatoes; return to a boil. Reduce heat to medium-high and simmer until the potatoes are tender when pierced with a fork, 15 to 20 minutes.
Drain well and pat the potatoes dry using a clean kitchen towel or paper towels; transfer to a large rimmed baking sheet.
Drizzle oil over the potatoes; sprinkle with garlic salt and pepper and toss to coat.
Spread the potatoes evenly on the pan. Using a sturdy glass jar or mug, gently push down on each potato to smash it to ½-inch thickness, keeping the potato intact.
Roast the potatoes on the lower rack until golden and crispy, about 25 minutes.
Remove from oven. Turn broiler to high.
Top the potatoes evenly with cheese and bacon; broil on the upper rack (about 6 inches from heat) until the cheese is melted, about 2 minutes.
Transfer to a platter; dollop with sour cream and sprinkle with chives. Serve immediately.
