Drain the green beans thoroughly in a colander to remove as much excess liquid as possible.
In a large bowl, whisk together the condensed cream of mushroom soup, whole milk, soy sauce, garlic powder, onion powder, and black pepper until smooth.
Lightly grease a 5- to 6-quart slow cooker. Add the drained green beans and mushroom sauce. Add about 3/4 cup of the crispy fried onions and gently stir until everything is evenly coated.
Cover and cook on LOW for about 4 hours or HIGH for 2–3 hours, until the casserole is hot and bubbling around the edges.
During the final 15–20 minutes of cooking, sprinkle about 3/4 cup crispy fried onions over the casserole. Replace the lid and finish cooking.
Just before serving, sprinkle the remaining crispy fried onions over the top for extra crunch. Serve warm.

Drain the green beans thoroughly to prevent the casserole from becoming watery.

Frozen green beans: About 2 pounds of frozen green beans can be substituted, although the texture and cooking time may vary.

Crispy onions: Keep most of the fried onions for the end of cooking so they stay crispy.

Slow cooker size: A 5- to 6-quart slow cooker works well for this recipe.

Make ahead: Combine the green beans and mushroom sauce ahead of time and refrigerate. Keep the crispy onions separate until cooking.

Storage: Refrigerate leftovers in an airtight container for 3–4 days.

Reheating: Reheat until thoroughly hot, then add fresh crispy fried onions for the best texture.