Preheat oven to 350F and grease an 8.5x11" casserole baking dish with avocado oil.
In a skillet, sauté the chopped onion in avocado oil for about 5 min, or until soft.
Add the chicken, rice, chopped herbs, mushrooms, and sautéed onion to a large mixing bowl and toss to combine. After all of the base ingredients have been thoroughly mixed, transfer to the baking dish.
Add all the ingredients for the sauce into a saucepan and blend with a hand immersion blender. Once the mixture is smooth, bring to a simmer and continue to stir for a few minutes until the sauce thickens. You may need to periodically blend the mixture if it becomes lumpy.
Remove sauce from heat and carefully pour over the base. Give it a good stir to evenly coat everything.
Bake for about 55 minutes, until the edges are golden brown. Let cool for about 5 minutes before serving.