Boil potatoes: Wash them well and place them into a pot. Cover them with cold water, bring to a boil, and cook them for 10-15 minutes or until fork tender. The exact cooking time depends on their size; check if they are cooked through before draining them.
In the meantime, preheat the oven to 400°F/200°C.
Drain the potatoes well and leave them to stand in the colander for about 5 minutes so that most of the water on their surface will evaporate.
Mixture: Mix olive oil, minced garlic, and finely chopped rosemary in a small bowl.
Smash fingerling potatoes: Grease the baking sheet with the extra olive oil. Place the potatoes on it and crush them carefully and only lightly with a fork; they should not be pressed flat but still have plenty of ridges after you've crushed them.
Drizzle the garlic mixture on each potato. Use about ½ teaspoon per potato, ensuring each one gets enough. If there's any leftover mixture, go over the potatoes again to evenly distribute it among them.
Bake: Sprinkle with salt and pepper and bake for 15-20 minutes or until golden and crispy. Increase the baking time slightly if the potatoes are not crisp enough after 20 minutes.
Remove them from the oven, sprinkle again with some coarse salt and pepper, and serve them immediately.