Beans: Preheat oven to 400°F (204°C). Bring a large pot of water to a boil and add a generous pinch of salt. Prepare a bowl of cold water with ice nearby. Add green beans to the boiling water and cook for 5 minutes, then transfer directly to an ice bath.
Mushrooms: Set a skillet over medium/high heat and heat 1 Tbsp of oil, then add mushrooms and garlic. Cook until mushrooms release their moisture, 5 to 7 minutes. Remove from the pan and set aside.
Sauce: Wipe out the skillet and set over medium heat. Add 4 Tbsp of oil, then stir in the flour. Cook for about 2 minutes, until flour loses its raw smell. Slowly whisk in the broth, then the almond milk, whisking constantly as you pour. Cook for 3 to 5 minutes, or until the sauce has thickened, then stir in salt, pepper, and nutmeg.
Assemble: Combine beans (strained), mushrooms, sauce, and a handful of the fried onions (½ cup) in a large mixing bowl. Transfer to a casserole dish.
Bake: Bake for 15 to 20 minutes, or until sauce is bubbling at the edges. Remove from oven and top with remaining fried onions. Bake for 5 minutes, or until crispy on top.