Preheat oven 375°.
Cook green beans in a pot over stovetop; once cooked, set aside.
While beans are cooking, make the crispy onions: heat butter over medium heat in a large, deep skillet and add the onions. Sauté onions for 3 minutes or until they start to soften, then add the breadcrumbs, reduce heat to medium-low and cook onions for an additional 3-5 minutes.
Use a slotted spoon to remove the onions and set aside. Use a paper towel to wipe out skillet (it does't have to be perfectly clean). Then, in that same skillet, heat olive oil over medium heat and add the mushrooms. Cook mushrooms until they're soft, 5-8 minutes.
While the mushrooms are cooking, make the cauliflower sauce.
To make the sauce: place cooked cauliflower, milk garlic, and salt in a blender and pulse until smooth; add pepper to taste.
Pour cauliflower sauce into skillet with mushrooms; stir in the broth, 1/4 cup almond milk and cornstarch and stir on medium-low heat until the sauce thickens. At this point, pour cooked green beans into skillet and mix everything together so that beans are coated with the sauce and mushrooms.
Grease a casserole dish (I used a 2.5 quart dish) with nonstick spray then pour the beans into the dish. Sprinkle onions overtop and bake for 25-30 minutes.
If you want the onions extra crispy, turn broiler on high and broil for 3-5 minutes before removing dish from the oven. Enjoy!