Preheat oven to 375°F. Boil green beans 5 minutes until bright but not soft. Drain and set aside.
Drain cashews. Blend cashews and plant milk until very smooth, about 2 minutes. Set aside.
Heat oil in a skillet over medium. Cook onion and mushrooms 6-8 minutes until soft. Add garlic and cook 1 minute.
Stir in flour and cook 1 minute. Slowly add cashew milk while stirring to thicken. Add soy sauce, salt, and pepper.
Mix green beans with sauce and pour into a 9x9-inch baking dish. Top with fried onions.
Bake 20 minutes until bubbling and the top is golden.