Preheat oven to 400 degrees F.
To roast the cabbage: lightly drizzle olive oil on 2 roasting sheets or cookie sheets covered with foil. Remove the bottom core of the cabbage carefully with a knife. Slice the head of cabbage into ¼ - ½ inch slices. Place on the roasting sheets and lightly drizzle the slices with more olive oil. Sprinkle salt, pepper, and garlic powder. Bake for 15 minutes until the cabbage looks wilted. Carefully with a spatula or tongs, remove the cabbage from the oven and flip so the cabbage can be evenly roasted. Bake for another 10-15 minutes until golden brown with some crispy edges.
In a large soup pot, sauté the chopped onion in olive oil until onions are translucent in color. Add the carrots and potatoes. Sprinkle with salt and pepper to flavor the vegetables, then add in Italian seasoning. Stir with a wooden spoon occasionally and sauté for about 10-15 minutes. Add in the celery and sauté for another 5 minutes. Add the chicken broth, chicken, and roasted cabbage; simmer for about 15 minutes on low-medium heat. All of the vegetables will be cooked by this step. Serve with warm crusty bread for a complete and cozy fall meal- enjoy!