CUT cauliflower into florets (about 3 cups); PLACE florets in a medium saucepan; ADD enough salted water to just cover; BRING to a boil; COOK until tender, about 10 minutes; DRAIN; SET aside.
In a medium saucepan, MELT butter over medium-high heat; WHISK in flour until smooth; gradually ADD milk, WHISKING constantly, until mixture begins to thicken.
ADD cheese and sour cream, STIRRING until cheese melts (do not boil).
STIR cauliflower, ham and mushrooms into cheese sauce; POUR into a 2-quart baking dish.
SPRINKLE breadcrumbs evenly over casserole; CUT cold butter into pieces; DISTRIBUTE evenly over breadcrumbs.
BAKE casserole, uncovered, at 350 F for 45 minutes.
Per Serving (excluding unknown items): 367 Calories; 26g Fat (64.4% calories from fat); 17g Protein; 16g Carbohydrate; 1g Dietary Fiber; 85mg Cholesterol; 885mg Sodium. Exchanges: 1/2 Grain(Starch); 2 Lean Meat; 1/2 Vegetable; 0 Non-Fat Milk; 4 Fat.