Preheat the oven to 350°F (175°C).
Lightly grease a 1 1/2-quart casserole dish or baking dish.
Drain the canned green beans thoroughly. Removing as much excess liquid as possible helps prevent a watery casserole.
Add the green beans and condensed cream of mushroom soup to a large microwave-safe mixing bowl.
Stir gently until the beans are evenly coated with the creamy mushroom soup.
Microwave on high for approximately 3–5 minutes, or until the mixture is warm.
Add 1/2 cup of the shredded Cheddar cheese and stir until evenly distributed.
Microwave for another 2–3 minutes, until the mixture is hot and the cheese begins to melt.
Transfer the mixture to the prepared casserole dish and spread it into an even layer.
Sprinkle the remaining 1/2 cup Cheddar evenly over the top of the Best Green Bean Casserole.
Finish with an even layer of crispy French-fried onions.
Bake for approximately 10 minutes, or until the filling is bubbling around the edges, the Cheddar has melted, and the onions are lightly golden.
Watch the casserole closely during the final few minutes because the fried onion topping can brown quickly.
Remove from the oven and allow it to rest for about 5 minutes before serving.