Preheat oven to 425 degrees F. Line a large baking sheet with parchment paper and spray with olive oil spray. Then grease a 9x13-inch baking dish with olive oil spray. Set both aside. 

 Fill a large saucepan with water and bring to a boil over high heat.  Add in your green beans and cook for 5 minutes. Drain beans and place in prepared baking dish. 

Meanwhile, while you are waiting for the water to boil and the beans to cook, make your Crispy Onion Topping. 

Place sliced onions in a large bowl and spray liberally with olive oil spray. Add in panko bread crumbs, garlic powder, salt and parmesan cheese. Use your hands to toss to combine, then place onions onto prepared baking sheet.  Spray everything with olive oil spray again and bake for 10-15 or until the breadcrumbs are golden and the onions are crispy. 

Next, prepare your mushroom alfredo sauce.  Heat a ceramic or stainless steel pan over medium heat.  Add in 2 teaspoons of butter, then add the mushrooms and sauté for 5 minutes, stirring occasionally, until lightly browned and soft.  Add garlic and the remaining butter, and sauté for 1-2 more minutes, stirring occasionally, until fragrant. Slowly whisk in flour until well combined. Slowly whisk in almond milk.  Increase heat to medium high and allow milk and flour to thicken for several minutes.  Once your roux is thickened, reduce heat to low and add in all the cheese, stirring well until the sauce is creamy. Remove from heat. Add in ¼ tsp of salt (or to taste), a sprinkle of pepper, and a pinch of nutmeg.  Mix in 1/4 cup of the crispy onion topping. 

Pour mushroom alfredo sauce over green beans, then top with the remaining crispy onion topping. 

Bake for 25-30 minutes or until everything is warmed and topping is golden brown.  (Keep an eye on it so that it does not burn.  If it does start to get too dark, simply lay a piece of aluminum foil on top of the casserole.)