Preheat the oven to 350°F (175°C). This allows the casserole to bake evenly and develop a golden, crispy topping.
Prepare the green beans:
If using canned green beans, open the cans and drain them well.
If using fresh green beans, wash, trim the ends, and cut them into bite-sized pieces. Then, steam or boil until tender (about 5-7 minutes), and drain them well.
Make the creamy base:
In a large mixing bowl, combine the cream of mushroom soup and milk. Stir until smooth. Season with black pepper, garlic powder, and salt (if desired). This forms the creamy sauce that coats the green beans.
Combine the green beans and soup:
Add the prepared green beans to the soup mixture and stir until evenly coated.
Assemble the casserole:
Pour the green bean and soup mixture into a 9x13-inch casserole dish and spread it evenly.
Top with French Fried Onions:
Open the can of Durkees French Fried Onions and generously sprinkle them on top of the casserole. Ensure they are spread evenly to create a crispy topping.
Bake:
Place the casserole in the oven and bake for 25 minutes. After 25 minutes, remove the casserole from the oven and stir gently. Return it to the oven and bake for an additional 5-10 minutes, or until the topping is golden brown and crispy.
Let it cool and serve:
Allow the casserole to cool for a few minutes before serving. This helps the casserole set and makes it easier to serve.