Preheat oven to 375 °F (190 °C). Lightly grease a 2.5-quart casserole dish (or something similar).
In a large pot, bring salted water to a boil and add green beans. Cook for 5 minutes. Remove from heat and rinse with cool water to stop them from cooking further (this will prevent soggy green beans). Drain well and set aside.
In a large pan, heat 1 tablespoon of oil over medium heat. Add onions and sauté until softened and translucent, about 3-4 minutes.
Add mushrooms and remaining 1 1/2 tablespoon of oil. Sauté until they release their moisture, are reduced in size and are lightly browned. About 5-7 minutes.
Add minced garlic, thyme, sage, nutmeg, black pepper. Sauté until fragrant, about 30-60 seconds.
Turn heat up to med-high and add the white wine. Once it simmers, lower back to medium heat. Cook until liquid almost completely reduces, about 1-2 minutes.
Add flour to the pan and stir constantly for 30-60 seconds. Now slowly add the vegetable broth while constantly stirring. Stir in nutritional yeast. Simmer until it thickens, about 2-3 minutes.
Turn heat to medium-low and add the coconut milk. Stir often to prevent burning. Cook for 2-4 minutes until thickened. Taste for salt and add more needed. Remove from heat.
Add green beans into the pan and gently fold to combine. Pour into the casserole dish. Bake uncovered for 15 minutes until heated throughout. Now cover the top with fried onions and cook for 5 minutes more or until hot and bubbly. Serve and enjoy!