Fill a large bowl with ice water and set it aside. This step is crucial for quickly cooling the green beans, which helps maintain their vibrant color and crisp texture after cooking. Ensure the bowl is large enough to fully submerge the green beans once they are blanched.
Fill a large pot with water and add enough salt to taste, ensuring the water is well-seasoned for flavor. Place the pot over medium-high heat and bring it to a rolling boil. Add the green beans and gently stir them occasionally to prevent sticking. Cook until they become tender yet still bright green, which should take about 2 to 3 minutes.
Be careful not to overcook as this can result in mushy vegetables that lose their crispness.
Use a slotted spoon to carefully transfer the cooked green beans from the boiling water into an ice bath. This abrupt temperature change stops the cooking process, ensuring the green beans remain crisp and vibrant in color. Allow them to cool completely for about 2 minutes before draining well. Ensure all excess water is removed to prevent diluting the casserole's sauce later.
In a large bowl, combine the mushroom soup, milk, pepper, and salt thoroughly to ensure no lumps remain. This step is crucial for achieving a smooth consistency in your casserole. Next, gently stir in the cooled green beans until they are evenly coated with the mixture. Ensure that each bean is well-coated to prevent any from sitting dry in the final dish.
Bake the casserole at 375°F (190°C) until it is heated through and just starting to bubble on the edges, about 30 minutes. This ensures that all ingredients are thoroughly cooked without overbaking. After baking, stir the mixture gently to redistribute any settled components.
Sprinkle the top with the fried onions in an even layer. Continue baking for another 5 minutes until the onions are golden brown and crispy. This step adds a delightful crunch and enhances the flavor of the casserole. Allow the casserole to cool slightly before serving to prevent burning your mouth.