Microwave the frozen cauliflower florets then the frozen broccoli florets until tender.
While the cauliflower and broccoli steam, cook the bacon slices in a large skillet until crispy.
Transfer the bacon to a paper towel-lined plate and crumble into pieces.
Stir together the shredded cheeses then, in a separate bowl, mix together the sour cream, cream cheese, mayonnaise, and spices.
Open the bags of steamed vegetables and pour out any extra water. Then, into the bowl of the cream sauce, fold in the veggies, half of the crumbled bacon, and about 1 cup of shredded cheese mixture.
Transfer the cream sauce mixture to the prepared baking dish and spread it into an even layer. Sprinkle the remaining shredded cheese mixture then the remaining bacon crumbles on top.
Cover the casserole dish with foil, transfer it to the oven, and bake. Then, remove the foil, move the oven rack to the top rack, and turn on the oven broiler. Broil the casserole until the cheese on top is bubbly and golden.
Remove the dish from the oven, and allow the casserole to cool slightly before garnishing with chopped green onions and serving.