Preheat the oven to 400°F.

Spray an 8- or 9-inch square casserole dish with nonstick cooking spray. Set aside.

Bring a large pot of salted water to a boil.

Cut the green beans into 1 1/2-inch pieces. Slice the mushrooms, finely chop the onion, and mince the garlic.

Once the water is boiling, add the green beans. Blanch until beans are bright green, about 3 minutes. Drain the green beans into a colander and rinse with cold water.

Set a large pot over medium-high heat and add the olive oil. Once oil is hot, add the onions and mushrooms. Saute until the mushrooms are browned and shrunken and the onions are soft, 6-7 minutes. Add the garlic and thyme and cook until fragrant, about 30 seconds.

Sprinkle the flour over the mushrooms and onions and stir with a spatula to coat. Slowly pour the vegetable broth into the pot, whisking to dissolve the flour. Pour in the nut milk, and season with salt and black pepper.

Cook the sauce, whisking constantly, until it begins to bubble and thicken. Reduce the heat to medium-low and allow the sauce to simmer, whisking occasionally, until it is reduced by about half, 5-7 minutes.

Add the green beans to the pot and stir to coat with sauce. Pour the green bean mixture into the prepared baking dish.

Bake the casserole on middle rack of oven for 15 minutes.

Open the oven and sprinkle the french fried onions in an even layer over the casserole. Continue baking until the casserole is bubbling at the edges, 5-10 minutes.

Check to see that casserole is done. Remove from oven or add time as needed.

Allow the casserole to rest for 10-15 minutes before serving.