A few simple steps can turn knobby celery root into crispy, oven-baked fries.
Preheat the oven to 450 degree F. On a large rimmed baking sheet, toss the celery root with the rosemary sprigs and 2 tablespoons of the olive oil. Season to taste with salt and pepper. Roast, tossing occasionally, until golden brown, 30 to 40 minutes.
In a food processor or blender, combine the garlic, lemon juice and 1/8 teaspoon salt and pulse several times. Let sit for 1 minute then add the egg and egg yolk and blend until combined. With the food processor or blender running, add the remaining 3/4 cup olive oil in a slow, steady stream. Season to taste with salt and pepper and more lemon juice, if necessary. If you want to make a spicy mayo, add in a little Sriracha of canned chipotle peppers.
Serve the celery root fries hot with the aioli for dipping.