Set your oven to 350°F and let it fully preheat. A hot oven helps the sauce bubble evenly from the edges to the center. I always grab my casserole dish now so nothing slows me down later.
If you use fresh beans, trim the ends, cut them into bite-size pieces, and cook them until just tender. Frozen beans should be thawed and canned beans should be rinsed. Pat or drain them well, because extra water makes the sauce runny.
Pour the mushroom soup into a mixing bowl. Add the milk, soy sauce, and black pepper. Stir until the sauce looks smooth with no streaks of soup left.
Fold the green beans into the sauce until every piece is coated. Stir in half of the crispy onions, which is about 2/3 cup. The onions soften inside and quietly boost the flavor.
Spoon the mixture into your 1 1/2-quart casserole dish. Spread it flat so it cooks at the same speed. Wipe any drips from the rim to avoid burnt edges.
Slide the dish into the oven and bake for 25 minutes. The sauce should bubble gently around the edges. Stir it once halfway if you like extra even heating.
Take the dish out and sprinkle the remaining onions over the top. Return it to the oven for 5 minutes to crisp them up. Watch closely, because onions can darken fast.
Let the casserole sit for 5 minutes before serving. The sauce thickens slightly as it cools. Then scoop it out and enjoy it while it is hot.