Prepare the Green Beans:
Bring a large pot of salted water to a boil. Add the green beans and blanch for 3-4 minutes until bright green and slightly tender.
Drain the beans and transfer them to an ice water bath to stop the cooking process. Once cooled, drain and set aside.
Make the Mushroom Sauce:
Heat olive oil or vegan butter in a large skillet over medium heat. Add the onion and garlic, sautéing until softened, about 3-4 minutes.
Add the mushrooms and cook until they release their moisture and start to brown, about 6-8 minutes.
Sprinkle the flour over the mushrooms and stir well to coat. Cook for 1-2 minutes to remove the raw flour taste.
Slowly whisk in the plant-based milk and vegetable broth, ensuring no lumps remain.
Add the soy sauce, thyme, salt, and pepper. Simmer the sauce for 5-7 minutes, or until thickened. Remove from heat.
Assemble the Casserole:
Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the blanched green beans with the mushroom sauce. Transfer the mixture to a greased casserole dish.
Prepare the Topping:
In a small bowl, mix the crispy fried onions, panko breadcrumbs, and melted olive oil or vegan butter until well combined.
Sprinkle the topping evenly over the green bean mixture.
Bake:
Bake the casserole in the preheated oven for 20-25 minutes, or until the topping is golden brown and the sauce is bubbling.
Serve:
Remove from the oven and let cool for a few minutes before serving. Enjoy this creamy, crunchy side dish with your favorite main courses.