Smashing parboiled potatoes creates more surface area and craggy edges that get crisp when roasted in a hot oven.
Position rack in lower third of oven; preheat to 450°F.
Place potatoes in a large pot, cover with water and bring to a boil. Cook, covered, until fork-tender, about 15 minutes. Drain.
Drizzle 1 tablespoon oil on a large rimmed baking sheet. Add the potatoes and, using the bottom of a glass coated with cooking spray, gently smash each potato to flatten. Drizzle with the remaining 1 tablespoon oil and sprinkle with salt and pepper. Scatter garlic on the pan.
Bake the potatoes, flipping once halfway, until crispy, about 20 minutes. Sprinkle with Parmesan and lemon zest.