Preheat the oven to 425 F. Line a large baking sheet with parchment paper.
Use a sharp knife to slice the celeriac into ½” strips. Add them to a bowl and toss with oil.
Spread the celeriac in a single layer on the prepared baking sheet. Make sure the strips aren’t overlapping.
Bake for 25-35 minutes. Check on the fries every 5-8 minutes after the 25 minute mark.
Meanwhile, whisk together the mayonnaise, garlic, 2 tbsp parsley and lemon juice. Season with salt and pepper.
Remove fries from the oven when they are browned and looking reduced in size. Sprinkle with sea salt and fresh parsley. Serve immediately with garlic dip.
