Steam broccoli and cauliflower flowerets for 6-8 minutes, or until crisp-tender; drain well.
Preheat oven to 350*. Lightly grease a large (at least 4-qt baking dish).
Arrange steamed, drained flowerets in prepared baking dish.
In a bowl, mix together mayonnaise, cheddar cheese, parmesan cheese, green onions, garlic, dijon mustard, salt, pepper, and red pepper flakes. Pour this over flowerets in the baking dish.