Peel and rinse the potatoes. Slice them lengthwise into thin slices, and then into thin sticks. Place the potato sticks in a bowl and cover them with cold water. Swish them around, then pour off the water and replace it with fresh water. Swish them around again and pour off the water, then cover with fresh water and let soak for 2 to 3 hours.
Make the seasoning: Mix the pepper, lemon zest, salt, garlic powder, mustard powder and onion powder in a large bowl and set aside.
When you're ready to make the fries, heat about 3 inches of oil in a heavy pot until a deep-fry thermometer registers 300 degrees F.
Drain the potatoes and lay them out on baking sheets lined with paper towels. Blot them with more paper towels to dry. Carefully drop half of the fries into the oil and cook until they start to soften, 2 to 5 minutes. Remove with a slotted spoon and drain on fresh paper towels. Repeat with the remaining fries.
Increase the oil temperature to 400 degrees F. Fry the potatoes in two batches again until golden brown and crisp, 2 to 5 minutes, returning the oil to 400 degrees F between batches. Remove the fries from the oil, add to the bowl with the seasoning and toss to coat. When all the fries are cooked, toss with the parsley.