Melt the butter in the large cast-iron skillet. Sauté the onions, carrots, and celery until soft, about 5 minutes, then transfer to the slow cooker.
Sautéed the sliced sausage until browned, about 8 minutes. Drain the sausage and transfer to the slow cooker, reserving 2 tablespoons of the rendered fat. Sauté the cut chicken pieces until lightly browned, then drain and transfer to the slow cooker.
Add the tomato sauce, Ro*Tel®, chicken stock, Creole seasoning, oregano, cayenne pepper, garlic, and bay leaves. Stir and cover. Cook on low about 5 hours. Stir in the rice and continue cooking on low for about 10 minutes, then turn heat down to warm for about 20 more minutes. Stir and serve in bowls with chopped green onions for garnish.