Preheat your oven to 375 degrees Fahrenheit. Heat a large pot of water over medium heat on your stovetop. Once boiling, add the green beans and blanch them for a few minutes so they are tender but still crisp. Remove the green beans from the water and run them under hot water to stop them from continuing to cook.
Heat your butter in a large skillet over medium heat. Once hot, add the onion. Saute for 5-7 minutes or until very soft. Add the garlic and stir until fragrant. Add the mushrooms and half of the thyme, pepper, salt and garlic powder. Stir for a few minutes until the mushrooms are tender.
Make a slurry with ¼ cup of the vegetable broth and the flour.
Pour the remaining vegetable broth into the skillet along with the almond milk, lemon juice, coconut aminos and tahini and stir. Once the mixture begins to bubble, add the slurry to the pan. Stir until the sauce thickens.
Add the blanched and drained green beans to the pan and coat the green beans in the sauce. Remove the pan from the heat. The sauce should be the consistency of a thick cream sauce. If the mixture is too thin, heat it over the stovetop for a little longer until it thickens. If it is too thick, add a little more broth.
Pour the green bean mixture into a 9”x13” baking dish. Top with crispy onions.
Bake in a 375 degree oven for 25-30 minutes until the onions are browned and the sauce is bubbly. Allow the casserole to rest for 10 minutes before eating.