Preheat your oven to 350°F (175°C).
Prepare the green beans:
Fresh: rinse, trim, and cut into 1-inch pieces.
Canned: drain thoroughly.
Frozen: thaw briefly before using.
If using fresh or frozen green beans, blanch them in boiling salted water for 3 minutes. Drain well and set aside.
In a large mixing bowl, combine the cream of mushroom soup, soy sauce, black pepper, and milk or water if needed to thin the mixture.
Stir in the green beans until evenly coated. Add half of the crispy fried onions and shredded cheddar cheese if using.
Transfer the mixture into a greased baking dish and spread evenly.
Bake for 25 minutes.
Remove from the oven and top with the remaining crispy fried onions.
Return the casserole to the oven and bake for another 5–10 minutes until golden and bubbly.
Serve the Easy Green Bean Casserole Recipe warm alongside roasted chicken, turkey, or beef dishes.