In a medium-sized pot, melt butter on medium heat.
Sauté the diced onion until translucent, then add garlic. Sauté for an additional 2 minutes.
Mix in the flour. Cook for 1 minute before whisking in the non-dairy milk and vegetable broth. Add in the seasonings (pepper, salt, and optional nutmeg.)
Bring to a simmer for about 3 minutes.
Add the green beans and sauce to a 3-quart casserole dish. Mix well. Top with the crispy fried onions.
Bake for 20-25 minutes, or until the casserole is bubbling and the onion topping is browned.