Preheat. Preheat the oven to 450°F and grease a 9x13 baking dish.
Prep chicken. Pat the chicken dry with paper towels and place breast-side up in the prepared baking dish.
Make butter rub. In a small bowl, mix the butter, rosemary, 1 teaspoon salt, ½ teaspoon black pepper, ½ teaspoon garlic powder, and ½ teaspoon onion powder until blended.
Season chicken. Rub the butter mixture over the chicken, loosening the skin and spreading some directly over the breast meat.
Stuff cavity. Place the onion, carrots, and celery inside the chicken cavity, packing gently.
Prep vegetables. In a large bowl, toss the baby carrots and potatoes with olive oil and the remaining ½ teaspoon salt, ¼ teaspoon black pepper, ½ teaspoon garlic powder, and ½ teaspoon onion powder.
Assemble. Arrange the vegetables evenly around the chicken in the baking dish.
Roast. Bake for 30 minutes, then reduce the oven temperature to 375°F and continue roasting for 40 to 50 minutes, until the chicken reaches 160 to 165°F and the vegetables are fork-tender.
Rest and serve. Let the chicken rest for 10 minutes before carving and serving.
Notes
Even cooking. Fully thaw the chicken before roasting to avoid uneven doneness.
Juicy meat. Rubbing butter under the skin helps keep the breast moist and flavorful.
Browning control. Tuck wing tips under the chicken to prevent over-browning.
Storage. Store leftovers in an airtight container in the fridge for up to 5 days and reheat before serving.