Break the cauliflower into sections.
Cook for 10 minutes in salted water.
Combine cauliflower with 2 tbsp., butter, onion, cheese, sour cream, and salt.
Spoon into 1 1/2 qt casserole.
Melt remaining butter and toss with breadcrumbs.
Sprinkle over cauliflower mixture.
Bake at 350 degrees for 30-40 minutes.