Pour 2 to 3 inches of water into a large saucepan and set a steamer basket inside, keeping the water below the basket. Leave approximately 4 to 5 inches between the water and basket bottom so the florets cook in steam rather than water.

Bring the water to a rapid boil over high heat, add the broccoli to the basket, and reduce the heat to medium. Keep the pan uncovered so the broccoli stays bright and does not become overly soft.

Steam the broccoli for 5 to 7 minutes, then move it to a medium bowl. A fork should enter the florets easily, while the thicker stems should still have a little firmness.

Add more water to the saucepan if needed and return the steamer basket, again checking that water stays below its base. Wait until the water is boiling rapidly before adding the cauliflower.

Add the cauliflower, reduce the heat to medium, and steam it uncovered for approximately 10 minutes. The florets are ready when a fork pierces them easily without turning them mushy.

Transfer the cauliflower to the bowl with the broccoli and leave the vegetables to cool. As they cool, steam escapes and helps prevent a thin, watery cheese mixture.

Heat the oven to 375 °F and lightly coat a 9x13 casserole dish with neutral-flavored cooking spray. The prepared dish helps the creamy vegetable mixture spread cleanly into the corners.

Stir the sour cream, softened cream cheese, room-temperature heavy whipping cream, mozzarella, and cheddar in a large mixing bowl. Keep mixing until no visible pieces of cream cheese remain and the binder looks smooth.

Add the cooled broccoli, cauliflower, optional sweet onion, crumbled bacon, salt, and freshly cracked black pepper, then fold gently. Stop when the florets are evenly covered and the bacon is distributed throughout the mixture.

Spoon the mixture into the prepared casserole dish and smooth it into an even layer. Leveling the surface helps the casserole warm evenly from the center to the edges.

Scatter the mozzarella and cheddar over the top, then cover the dish with aluminum foil. The foil protects the cheese while the vegetables and creamy center heat through.

Bake the covered casserole for 20 minutes. The cheese should be fully melted, with gentle bubbling visible around the perimeter.

Remove the dish from the oven and let it stand for 5 minutes before serving warm. The brief rest lets the creamy binder settle so portions hold together more neatly.