Preheat oven to 400F and bring a large pot of water to a boil. Add whole or cut in half green beans and let cook for 5 minutes. Place in an ice bath to stop cooking, drain, and set aside.

In a large skillet over medium heat, cook mushrooms in melted butter. Season with salt and pepper and let cook for about 5 minutes. Add garlic and let cook until fragrant, about a minute or so.

To make the creamy sauce, stir in the flour and fully coat the mushrooms. Stream in the broth and bring to a simmer, stirring, until the mixture thickens.

Reduce heat to low, stir in dairy-free milk, and cook for about 5 minutes, until desired consistency is reached. Taste and adjust seasoning, adding more salt or pepper, if needed. Add cooked, tender green beans to the mushroom sauce and stir to combine.

To assemble green bean casserole, transfer mixture to a greased 9x13 baking dish (or similar size) and top with desired amount of crispy onions. Bake for 15 minutes, until top is golden, serve warm.