
Preheat the oven to 425°F.
In a large pot of salted boiling water, cook the potatoes until fork-tender, about 15–20 minutes. Drain and let the potatoes cool slightly.
Transfer the potatoes to a large rimmed baking sheet. Use a potato masher or the bottom of a measuring cup or glass to press down on the potatoes to smash them.
Drizzle the potatoes with olive oil, then add the garlic, rosemary, Parmesan, salt, and pepper. Toss with your hands to combine.
Bake for 15 minutes until the bottoms are golden. Use a spatula to flip the potatoes and add more salt, pepper, and olive oil, if desired. Return to the oven for 12–15 more minutes until the potatoes are crispy.