Preheat the oven to 350ºF and adjust the rack to the middle position. Melt the butter in a large sauce pan over medium-low and add the onions. Cook the onions, stirring occasionally, until caramelized, about 20 minutes. Add the mushrooms and salt and cook for 10 minutes, stirring occasionally. Add the chilis and stir to combine. Then, stir in the arrowroot followed by the broth, cream, tamari and tomatoes. 

Pour the entire mixture into a 9×13” baking dish. Add the rice and stir to combine. Place the four chicken breasts in the mixture and push them down so they are submerged in the rice and tomatoes. Cover with parchment and then foil (I do this so the foil doesn’t touch and leach aluminum into the food) and then bake for 50-60 minutes until the rice and chicken are cooked through.