Preheat oven to 450 degrees F (230 degrees C). Grease a large baking sheet with a rim.
Mix onion, vegetable oil, rosemary, thyme, salt, and pepper in a large bowl; add potato pieces and toss to coat. Spread potatoes into prepared pan in a single layer.
Bake, turning occasionally, until potatoes are lightly browned and tender when pierced with a fork, 20 to 25 minutes.
