Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius) and line a rimmed baking sheet with parchment paper.
Place fingerling potatoes in a large pot and cover with cold water by 1 inch. Add kosher salt.
Bring to a boil over high heat, then reduce to a simmer and cook for 15 to 20 minutes until potatoes are fork-tender.
Drain the potatoes thoroughly and allow them to air-dry for 5 minutes to remove excess surface moisture.
Distribute potatoes across the prepared baking sheet. Using a heavy glass or potato masher, apply vertical pressure to each potato until flattened to a thickness of approximately 0.5 inches.
Drizzle olive oil evenly over the smashed potatoes.
Season with sea salt, black pepper, rosemary, and minced garlic.
Roast in the preheated oven for 20 to 25 minutes, flipping the potatoes at the halfway mark, until the edges reach a deep golden brown and crispy texture.
Serve immediately to maintain structural crispness.