Preheat oven 375°.
Cook green beans in a pot over stovetop; once cooked, set aside.
While beans are cooking, heat butter in large skillet and add onions; stir. Once onions start to turn golden in color, add breadcrumbs and stir well. Cook for an additional 3-5 minutes and then set onions aside.
In the same skillet you used to cook the onions, heat olive oil and cook mushrooms.
While mushrooms are cooking, make cauliflower sauce.
To make the sauce: place cooked cauliflower, milk and garlic in a blender and pulse until smooth; add salt and pepper to taste.
Pour cauliflower sauce into skillet with mushrooms; stir in the broth, additional almond milk and flour and stir on low-medium heat until sauce becomes thicker. At this point, pour green beans into skillet and mix everything together so that beans are coated with sauce and mushrooms.
Grease a casserole dish with nonstick spray or coconut oil and add beans; pour onions overtop and bake for approximately 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.