Boil the potatoes. Thoroughly wash the potatoes and remove any bad spots (no need to peel the potatoes). Boil the potatoes in a large pot of water for 20 minutes, until fork-tender. Drain and let cool slightly.
Preheat the oven to 450°F. Brush two large baking sheets with olive oil. Spread the potatoes on the baking sheets, allowing space between each potato.
Spray the bottom of a sturdy glass with oil or cooking spray, then smash the potatoes until they're approximately ½ inch thick.
Make the topping. In a small saucepan, melt the butter. Remove from heat, then stir in the minced garlic and chopped rosemary and mix until combined. Generously brush the potatoes with the butter mixture, then sprinkle with salt and pepper.
Bake for 25-30 minutes until the edges are crispy (you do not need to flip the potatoes). Remove from the oven and serve while warm.