Cut potatoes into French fry-sized strips. Soak in a large bowl of cold water for about 30 minutes. Drain, then pat completely dry on a clean tea towel.
Preheat oven to 200 degrees C. Line two large oven trays with baking paper. Combine garlic powder, onion salt, paprika and parmesan. Place potatoes in a large bowl. Add seasoning and toss until evenly coated. Spread over prepared trays.
Spray with olive oil. Bake for about 35 minutes, turning fries over twice during cooking, until golden and crisp.
Serve sprinkled with salt and parsley.