Preheat the oven to 375℉.
Fill a large bowl with cold water and ice and set aside.
Fill a large pot with water and bring to a boil. Once boiling, add the green beans and cook for 3 minutes.
Using a slotted spoon to transfer beans to the ice bath to stop the cooking process. Once cool, remove from the ice bath and add to a 2 ½-quart oval baking dish. Set aside.
In a large skillet over medium heat, add the butter. Once melted, add the garlic, mushrooms, salt and pepper and saute until the mushrooms are tender, about 4 minutes.
Next, add the cassava flour and stir until well combined (it will be very thick). Then slowly add ½ cup of the chicken broth and stir to combine, scraping up any browned bits from the bottom of the pan. Next add the final ½ cup of the broth and stir to combine.
Add the lemon juice, dijon, onion powder, dried thyme, and cayenne (if using) and stir to combine.
Next, stir in the coconut milk until it is well incorporated and slightly thickened, about 2-3 minutes. Pour the mushroom mixture over the green beans and toss until well coated.
Place into the oven and bake for 10 minutes.
Carefully remove from the oven and toss once more. Add the crispy onions and spread evenly over the green beans.
Place back into the oven and bake for another 5-7 minutes, or until the onions are slightly browned, being careful not to burn.