Preheat the oven to 450 degrees F. Line a baking sheet with foil.
Place frozen fries in a large bowl and add the duck fat, using your hands to coat evenly. You don’t want a super thick coating, just a light glaze. Any excess with drip off, though, so don’t worry too much about overdoing it.
Arrange french fries in a single layer on the baking sheet. Bake according to package instructions, typically 10-15 minutes, gently flipping the fries once to ensure even cooking.
Remove from oven and allow to cool briefly. If necessary, carefully use a dishtowel to pat away excess duck fat. If using, sprinkle truffle salt on top or serve with truffle aioli.