Preheat the oven to 425°F. Position an oven rack in the lower-middle portion of the oven. Lightly grease a very large metal roasting pan or rimmed baking sheet with high sides.
If the chicken has not already been prepared, place it breast-side down on a sturdy cutting board. Using sharp kitchen shears, cut along both sides of the backbone and remove it. Flip the chicken breast-side up and press firmly on the breastbone until the chicken flattens. Pat the chicken thoroughly dry with paper towels.
In a small bowl, combine the softened butter, garlic powder, orange zest, lemon zest, ground coriander, Great North Maple Garlic Seasoning (or homemade substitute), paprika, kosher salt, black pepper, and dried parsley. Mix until smooth and evenly combined.
Carefully loosen the skin over the chicken breasts and thighs without tearing it. Spread approximately two-thirds of the citrus herb butter underneath the skin, distributing it evenly over the meat. Rub the remaining butter over the outside of the chicken, including the legs and wings.
Arrange the thick cabbage steaks and red onion wedges in the center of the roasting pan, creating a bed for the chicken. Drizzle with 1 tablespoon olive oil and season with approximately ½ teaspoon kosher salt and a pinch of black pepper.
Arrange the potatoes and garlic surrounding the cabbage and onions. Spread into a single layer as much as possible. Drizzle with a little more olive oil and kosher salt.
Place the spatchcocked chicken breast-side up directly on top of the cabbage and red onions. Tuck the rosemary and thyme sprigs around the chicken and vegetables. Squeeze lemon and orange juice over the chicken and then arrange the halved lemon and orange cut-side up in any available spaces around the pan.
Transfer the pan to the preheated oven and roast for approximately 55–75 minutes, depending on the size of the chicken and the roasting pan. Begin checking the internal temperature around 50 minutes. The chicken is fully cooked when the thickest part of the breast and thigh reaches at least 165°F, without touching bone. For especially tender dark meat, the thighs can reach approximately 175–185°F. If the exposed vegetables begin browning too quickly, loosely cover those areas with foil.
Once the chicken reaches the proper internal temperature, carefully transfer it to a large cutting board and allow it to rest for 10–15 minutes before carving.
Check the potatoes for tenderness. If needed, return the roasting pan to the oven for another 10–15 minutes, increasing the temperature to 450°F to encourage additional caramelization. The cabbage underneath the chicken should be tender and buttery, while the exposed edges develop a deeper roasted flavor.
Squeeze a little roasted lemon over the cabbage and onions, if desired. Spoon some of the pan drippings over the vegetables. Carve the chicken and serve with the roasted cabbage, red onions, potatoes, and carrots. Garnish with fresh rosemary or thyme.