In your crockpot combine the cream of mushroom soup, sour cream, milk, and seasonings, and whisk to combine.
Stir in the green beans, cheddar, parmesan, and half of the french fried onions. Place the lid on your crockpot and cook on low for 2 hours.
After two hours stir the casserole well and add the remaining french fried onions to the top. Replace the lid and continue cooking for 30 minutes, up to one more hour.
NOTE: The cheese in this recipe is optional, you can leave it out if desired without making any other adjustments.MAKE AHEAD: This casserole can be mixed up in a large bowl, covered, and stored in the refrigerator for up to 3 days before cooking.NOTE: After cooking this casserole can be left in the crockpot on warm for up to 2 hours.NOTE ON GREEN BEANS: If you are using fresh or frozen green beans instead of canned I recommend using a total of 24 ounces or 1.5 pounds total. If using frozen green beans, thaw them first.