In a large skillet, melt butter over medium heat. Add onions and cook until softened (about 3-4 minutes).
Add garlic and cook for 30 seconds. Add mushrooms and cook until softened (about 5 minutes). Stir in flour.
Slowly pour in vegetable broth followed by the oat milk. Simultaneously stir the mixture as you are pouring in the liquids. Add soy sauce.
Bring to a boil and simmer for about about 5 minutes until thickened. Add a few cracks of fresh black pepper, taste and adjust seasonings if needed.
Pour frozen green beans (no need to thaw) in a casserole dish (mine is a 2.5 qt). Mix well.
Top with crispy onions and bake for 20 minutes covered, then 10 minutes uncovered. Let cool for 10 minutes before serving.
Leftovers will keep well for up to 4 days covered in the refrigerator.To keep the onions crunchy I recommend you bake the casserole to reheat. 15 minutes in a preheated 350 degree F oven ought to do it.