It’s best to get the cauliflower in the steamer before cooking the vegetables and beef. This way all of the ingredients will be ready at the same time to bake.
Steam the cauliflower until tender. (If you don’t have a steamer then fill a large stockpot with water and add cauliflower. Simmer cauliflower until tender. Drain in a colander.) Place cauliflower, butter, sour cream, milk, and salt in the bowl of a food processor (you can also do this in a blender). Process until smooth. Season to taste with salt pepper. Spread pureed cauliflower over beef layer in baking dish. Sprinkle with cheddar cheese. Bake for 30 minutes or until bubbling and cheese is golden brown. Let cool for 10 minutes before serving.