Bring a large pot of water to a boil. Add green beans and cook for 5 minutes. Drain and place in an ice water bath to stop cooking. Drain and set aside. (Skip if using frozen green beans.)
Heat oil in a large oven-safe skillet over medium heat. Add onion, garlic, and mushrooms. Cook on medium-low heat until onions become transparent.
Sprinkle flour. Whisk to stir and coat vegetables. Cook for 1 minute. Slowly whisk in stock to create a sauce.
Preheat oven to 400°F or 205°C.
Add dairy free milk. Whisk to stir again. Reduce heat to low. Cook until it thickens into a sauce.
Add salt to taste. Remove from heat.
Place green beans in a casserole dish, pour sauce over them, and add 1//2 cup fried onions. Toss to coat well, and top with remaining fried onions.
Bake for 15 minutes or until lightly browned on top. Serve immediately.