Preheat oven to 450°F Coat a large rimmed baking sheet with nonstick spray. Toss potatoes with 2 1/2 tablespoons oil in a large bowl and season with salt and pepper. Arrange in a single layer on baking sheet.
Roast the potatoes, turning occasionally, until browned and tender, about 30 minutes. Increase heat to 500°F Continue roasting until fries are deep brown in spots, about 5 more minutes.
Whisk remaining 1/2 tablespoons oil, garlic, and parsley in a large bowl. Add hot fries, season with salt and pepper, and toss to coat.