- Preheat oven to 375ºF. Bring 8 cups of water to a rolling boil in a large stock pot and season with ½ teaspoon of sea salt. Add green beans. Boil for 3-4 minutes. Drain and blot beans dry with a clean kitchen towel. Spread beans evenly in a medium casserole dish greased with cooking spray.
- Heat olive oil over medium heat in a large skillet. Add onions and cook for 10 minutes, stirring regularly. Melt 2 tablespoons of butter in pan and add sliced mushrooms. Stir well and cook 10 minutes longer. Remove from heat and discard any excess liquid from the pan. Stir in garlic, thyme and ½ teaspoon of sea salt and cook for 2 minutes longer.
- Sprinkle onion and mushroom mixture over green beans.
- Melt remaining 2 tablespoons of butter in pan and sprinkle in flour, whisking to combine. Gradually add almond milk and tamari until thoroughly mixed. Add black pepper and nutmeg. Continue to heat until mixture has thickened, about 5-7 minutes. Pour sauce over green beans evenly. Cover casserole tightly with aluminum foil and bake for 35 minutes.
- Microwave 2 tablespoons of butter in a heat safe bowl. Add panko bread crumbs and stir to coat. Remove casserole from oven and discard foil. Increase oven temperature to 425ºF. Sprinkle panko over casserole and bake for 12-15 minutes longer, until panko is golden brown. Remove from oven and cool for 5 minutes before serving.