1. Preheat the oven to 425°F (220°C).
2. Roughly chop the carrots and fennel into bite-sized cubes.
3. In a small bowl, combine oats, 1/2 chopped onion, sage, salt, pepper, and some olive oil to create a paste-like rub.
4. Place the potatoes, chopped carrots, and fennel in a large cast-iron skillet or roasting pan.
5. Drizzle the vegetables with olive oil, season with salt, and toss to coat evenly.
6. Pat the chicken dry. Generously apply the oat stuffing inside the cavity.
7. Place the prepared chicken on top of the bed of vegetables in the skillet.
8. Drizzle the chicken with a little more olive oil and season with salt.
9. Roast in the preheated oven for approximately 1.5 hours, or until a meat thermometer inserted into the thickest part of the thigh registers 165°F (74°C) and the juices run clear.
10. Remove from the oven, let the chicken rest for 10-15 minutes before carving and serving with the roasted vegetables and stuffing.
