Bring a small pot of water to a boil, then place a strainer over the top. Add the green beans to the strainer, cover, and steam for 10-15 minutes, or until fork-tender. Set aside.
Heat a drizzle of olive oil in a medium pot over medium heat. Once the oil begins to shimmer, add the onion. Cook for 2 minutes, or until semi-translucent.
Add the mushrooms and cook for 5-6 minutes, until they have released the majority of their juices. Add 2 tablespoons of olive oil, the garlic, and soy sauce. Cook for 2-3 minutes, until the garlic is fragrant.
Stir in the flour to make a roux, then season with salt and pepper.
While stirring continuously, pour in the non-dairy milk and bring to a boil. Cook for 4-5 minutes, until the sauce has thickened.
Stir in the green beans and ½ cup (75 g) of the fried onions, until combined. Transfer to a medium baking dish and sprinkle on the remaining cup of fried onions.