Preheat oven to 350 degrees.
Break cauliflower into sections and cook for 10 minutes in boiling water. Drain well. Combine cauliflower with 2 tablespoons butter and onion, cheese, sour cream and salt. Spoon into a 1½ quart casserole.
Melt remaining butter and toss with bread crumbs. Sprinkle over cauliflower mixture. Bake at 350 degrees for 30 minutes or until heated through.