Lemon and Parsley Skillet-Roasted Fingerling Potatoes

Combine potatoes, lemon juice, kosher salt, and 3 Tbsp. butter in a large straight-sided skillet or a Dutch oven; add cold water just to cover. Bring to a boil over medium-high heat and cook until potatoes are tender, 20–25 minutes. Increase heat and boil rapidly until pan is almost dry, 15–20 minutes more (there will still be a sheen of butterfat).
Using a slotted spoon, transfer about half of potatoes to a plate and arrange potatoes remaining in skillet cut side down in a single layer (if your skillet is smaller, you may need to work in a few batches). Reduce heat to medium and cook potatoes undisturbed until water is evaporated and cut sides of potatoes are deeply browned, 5–10 minutes.
Using a thin metal spatula, scrape skillet to cleanly release potatoes and transfer to a large bowl. Return skillet to medium heat, add 1 Tbsp. butter, and repeat browning process with reserved potatoes. After scraping skillet to release potatoes, return first batch, along with parsley and remaining 2 Tbsp. butter, to skillet and toss until potatoes are well coated. Serve sprinkled with sea salt.
Potatoes can be boiled 4 hours ahead. Let cool in skillet, then cover and let sit at room temperature. Brown just before serving.