Heat the oven.Set it to 375°F (190°C). Grease your baking dish and set it aside.
Cook the green beans.Boil a big pot of salted water. Drop in the beans and cook for 3–4 minutes — just until they turn bright green and you can poke them with a fork. Drain them, then dump them into cold water to stop the cooking. Drain again and dry them off.
Make the crispy onions.Toss the sliced onions with flour, garlic powder, and salt. Heat oil in a pan over medium-high heat. Cook the onions for about 8–10 minutes, stirring now and then, until they're golden and starting to crisp up. Set them aside on a paper towel.
Start the sauce.In the same pan, heat a little oil over medium heat. Add the diced onion and cook for 4–5 minutes until soft. Add the garlic and stir for one more minute.
Add the mushrooms.Throw in the sliced mushrooms. Let them sit without stirring for 2 minutes so they get a little color, then stir and cook for another 2–3 minutes.
Make it creamy.Sprinkle flour over everything and stir to coat. Cook for one minute. Slowly pour in the oat milk and broth, stirring as you go. Add soy sauce, onion powder, and paprika. Let it simmer for 5–7 minutes, stirring often, until the sauce thickens up. Add salt and pepper to taste.
Put it together.Stir the green beans into the sauce until everything is coated. Pour it all into your baking dish and spread it out evenly.
First bake.Put it in the oven, uncovered, for 20 minutes — until the sauce is bubbling at the edges.
Add the crispy onions.Take it out, scatter the onions on top, and bake for another 5–8 minutes. Keep an eye on it — you want them golden, not burnt.
Let it rest.Leave it alone for 5 minutes before serving. The sauce will settle and thicken up a little.
