Make the compound butter: In a small bowl, combine the softened butter, minced garlic, onion powder, paprika, rosemary, thyme, and salt. Mix until smooth. Set aside.
Pat the chicken dry: Pat the whole chicken dry with paper towels. Dry skin is key for crispiness.
Add butter under the skin: Carefully lift the skin on the breast and thighs. Slide the compound butter underneath and spread it evenly across the meat.
Butter the cavity: Rub any remaining compound butter inside the cavity of the chicken.
Make the seasoning rub: In a large bowl, mix the oil, paprika, garlic powder, onion powder, salt, and pepper until combined.
Season the outside: Place the chicken in the bowl and rub the seasoning mixture all over the outside, including the legs and wings.
Preheat the oven: Preheat to 425°F.
Prepare the vegetables (optional): If adding vegetables, scatter the baby potatoes, baby carrots, and sliced red onion across the bottom of the casserole dish or roasting pan. Drizzle with a little oil and season with salt and pepper. Toss lightly to coat.
Roast: Place the chicken breast side up in a casserole dish or roasting pan. Truss if desired. Roast for 1 hour to 1 hour 20 minutes, until an instant-read thermometer inserted into the thickest part of the thigh (not touching the bone) reads 160–165°F. If the skin starts to get too dark, cover loosely with foil.
Rest: Remove from the oven and let rest for 10–15 minutes before carving.
Serve: Spoon the pan juices over the chicken before serving.