Preheat your oven to 450 degrees F and position a rack in the upper third. Drizzle a rimmed sheet pan generously with olive oil, enough to coat the entire surface, and set it aside.

Add the potatoes to a large pot and cover with cold water by at least 2 inches. Add a big pinch of kosher salt. Bring to a boil over high heat, then reduce to a steady simmer and cook for 15 to 18 minutes, until a fork slides into the center of the largest potato with no resistance.

Drain the potatoes and spread them in a single layer on the oiled sheet pan. Let them sit uncovered for 5 minutes so the steam escapes and the surfaces dry out a bit. This step is what helps them crisp instead of steam in the oven.

Use the flat bottom of a measuring cup or a sturdy glass to press each potato down firmly until it is about 1/2 inch thick. The skins will split and the potato will spread out, which is exactly what you want.

In a small bowl, stir together the 3 tablespoons of olive oil, minced garlic, chopped rosemary, and black pepper. Drizzle this mixture evenly over all the smashed potatoes, letting it pool in the crevices.

Roast on the upper rack for 20 to 25 minutes, until the edges are deep golden brown and visibly crispy. Do not flip them. The undersides will crisp against the oiled pan on their own.

Pull the pan from the oven and immediately sprinkle with flaky sea salt. Taste one and adjust salt as needed. Serve hot straight from the pan.