Steam broccoli and cauliflower flowerets for 6 to 8 minutes or until crisp tender. Drain well.
Arrange flowerets in prepared baking dish. Stir together: mayonaise, cheddar cheese, parmesan cheese, green onions, and dijon mustard. Spoon over flowerets. Sprinkle with breadcrumbs.
Bake for 20 to 25 minutes or until golden.