Prep. Preheat oven to 350ºF. Lightly grease a 9x13 casserole dish with cooking spray and set aside.
Cook the beans. Fill a saucepan with water and bring to a rapid boil. Add green beans to the boiling water and cook for 3 minutes, or to your desired tenderness. DON'T overcook, the beans will be mushy if you do. Rinse the beans under cold water until they're completely cooled, then drain thoroughly and place in a large mixing bowl. Set aside.
Sauté the veggies. In the meantime, melt 3 tablespoons of butter in a large skillet. Add onions to the skillet and season with a pinch of salt. Cook over medium heat for 3 minutes, or until soft, stirring occasionally. Add the mushrooms, turn up the heat to medium-high, and continue to cook for 4 minutes, stirring occasionally.
Combine the veggies. Remove from heat and transfer the mushroom mixture to a large mixing bowl with the green beans. Set aside.
Make a roux. In the same skillet, melt 3 tablespoons butter over medium heat. Whisk in flour; continue to whisk for about 1 minute, or until the flour mixture turns light brown.
Make the sauce. Slowly pour in the soy milk and continue to whisk and cook for 4 minutes, or until thickened. Season with salt, pepper, and nutmeg; remove from heat. Taste for seasoning and adjust accordingly. Let stand for 5 minutes.
Put it all together. Stir the sauce and pour it over the green beans and mushrooms; toss to combine and coat evenly. Transfer the green beans mixture to the previously prepared casserole dish.
Bake. Bake the casserole for 30 to 35 minutes or until the sauce bubbles. Remove from oven and let stand for several minutes.
Add the onion topping. Give it a good stir and sprinkle over crispy fried onions for serving.
Notes
Nutritional information does not include the crispy onions topping.
Store leftovers airtight in the fridge and reheat within 3 days.
Freeze tightly wrapped in plastic wrap or foil, or in a freezer-safe container for up to 4-6 months.