This is a casserole my grandmother would serve using her leftover ham from Easter. I also make it to use up leftover ham, or I buy a ham slice and make it anytime because so many people like it. Enjoy!
Cut cauliflower into florets (about 3 cups). Cook in boiling salted water to cover 10 to 12 minutes, or until tender; drain and set aside.
Melt 4 tablespoons butter in a medium saucepan over medium-high heat. Whisk in flour until smooth. Gradually add milk, whisking constantly, until mixture begins to thicken. Lower heat to medium. 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 1 tablespoon butter into pieces and sprinkle evenly over breadcrumbs.
Bake casserole, uncovered, at 350 degrees for 45 minutes.