Preheat your oven to 350°F (175°C). Lightly grease a 1 1/2- to 2-quart baking dish with cooking spray or a bit of butter.
If using fresh green beans, bring a pot of salted water to a boil.
Blanch beans for 4–5 minutes until crisp-tender, then drain and rinse under cold water to stop the cooking.
In a large bowl, stir together the cream of mushroom soup, milk, soy sauce, and black pepper until smooth. If using optional seasonings like garlic powder or cheese, mix them in now.
Add the green beans and about half of the French-fried onions. Stir gently to coat everything evenly without crushing the onions too much.
Transfer the mixture to your prepared baking dish.
Spread it into an even layer so it cooks uniformly.
Bake for 25–30 minutes, until the edges are bubbling and the center is hot.
Remove from the oven and gently stir the casserole once. Sprinkle the remaining French-fried onions over the top in an even layer.
Return to the oven and bake for another 5–8 minutes, just until the topping turns golden and crisp. Keep an eye on it so it doesn’t darken too much.
Let it rest for 5 minutes before serving.
The sauce thickens slightly as it cools, making for cleaner scoops.