Preheat oven to 350 degrees. Grease a 9 x 13 casserole dish with olive oil or butter.
Heat a large sauté pan to medium-high heat and add 1 tablespoon olive oil. Turn pan to coat with oil. Add sliced onions and allow to brown without stirring, for 5-6 minutes. You want to burn the edges slightly. Flip onions with a spatula and cook another 5-6 minutes to brown the other side. Add 1/2 cup water to the pan to deglaze, scraping the bottom and mixing it with the water. Turn down to medium-low heat and cook another 10 minutes.
Remove onions from pan and set aside. Add 2 more tablespoons olive oil to pan, turn to medium heat and add sliced mushrooms. Cook 3-4 minutes without stirring to brown slightly, then flip and cook another 2-3 minutes. Add minced garlic and saute 30 seconds, stirring.
Add coconut milk, chicken stock, mustard, tamari, salt, pepper, garlic powder, onion powder and oregano. Stir to combine. Cook for 15 minutes or until the liquid has reduced by approximately half and has thickened.
Whisk arrowroot starch with 6 tablespoons water and add to sauté pan. Stir to incorporate and cook another 2-3 minutes until liquid has thickened even more. Remove pan from heat.
Add steamed green beans and caramelized onions to casserole dish and pour mushroom mixture over. Stir to incorporate evenly. Top with Panko mixture. Optional but recommended: drizzle top with extra virgin olive oil. Bake for 30 minutes. Allow to cool 10 minutes before serving.