Trim the Brussels sprouts, remove damaged outer leaves, and halve them lengthwise. Quarter any especially large sprouts so the pieces cook at the same rate.

Cook the chopped bacon in a large oven-safe skillet over medium heat for 6 to 8 minutes, or until browned and crisp. Transfer it to a paper-towel-lined plate, leaving 1 to 2 tablespoons of bacon fat in the skillet.

Place the Brussels sprouts cut-side down in the skillet and cook for 4 to 6 minutes, stirring occasionally, until lightly browned. If the skillet is crowded, work in batches and add 1 tablespoon unsalted butter if more fat is needed.

Add the diced onion and cook for 2 to 3 minutes until softened. Stir in the minced garlic and cook for about 30 seconds, just until fragrant.

Return all the Brussels sprouts to the skillet and pour in the heavy cream. Season with kosher salt, black pepper, and optional red pepper flakes or Italian seasoning, then bring the mixture to a gentle simmer.

Reduce the heat and simmer for 5 to 8 minutes, stirring occasionally, until the sprouts are tender and the cream has thickened slightly. Cover for 2 to 3 minutes if you prefer especially tender sprouts, then uncover so the sauce can reduce.

Reduce the heat to low and stir in the Parmesan and mozzarella until melted and smooth. Fold in most of the cooked bacon, reserving some for the finished dish.

For a browned top, place the oven-safe skillet in a 400°F oven for 5 to 10 minutes, or broil briefly until bubbling and golden. Watch closely under the broiler because the cheese can brown quickly.

Scatter the reserved bacon over the top and serve hot. Spoon plenty of sauce over each portion so every serving includes tender sprouts, melted cheese, and crisp bacon.