In medium bowl, whisk together salad dressing mix, sour cream, and milk.
Drizzle 3/4 cup dressing mixture over broccoli (reserve remaining dressing for another use).
Top with cheese.
Cover loosely with foil.
Bake until heated through 15 to 20 minutes. If desired, combine breadcrumbs and butter. Sprinkle on top of casserole during last 5 minutes of baking; do not cover with foil.