Prepare the slow cooker base: Lightly grease a 5- to 6-quart slow cooker with a bit of oil or nonstick spray. This helps prevent sticking and makes cleanup easier.
Mix the creamy sauce: In a medium bowl, whisk together the condensed cream of mushroom soup and sour cream until smooth. This mixture will create the rich, creamy sauce that bubbles up through the casserole and helps form that golden, crusty top.
