Prepare the Baking Dish:

• Preheat oven to 400°F. Lightly grease an oval gratin-style baking dish with butter, oil, or cooking spray.

Blanch the Brussels Sprouts:

• Prepare an ice bath. Bring a large saucepan of salted water to a boil (about 2 tbsp salt).
• Drop in the Brussels sprouts and cook for 3-4 minutes until bright green and slightly tender.
• Drain and transfer to the ice bath. Drain again and pat dry with a kitchen towel.

Cook the Bacon:

• In a deep skillet, cook the diced bacon over medium heat until crispy.
• Remove bacon and drain on a paper towel-lined plate, reserving the bacon drippings in the pan.

Make the Cheese Sauce:

• Whisk flour into the reserved bacon drippings and cook for 1-2 minutes over low heat.
• Stir in heavy cream and bring to a simmer over medium-low heat, cooking until slightly thickened (2-3 minutes).
• Gradually add shredded cheese, whisking until melted and smooth.
• Season with salt, black pepper, and cayenne (if using).

Assemble the Gratin:

• Stir Brussels sprouts and bacon into the cheese sauce, coating evenly.
• Transfer mixture to the prepared baking dish.

Make the Topping:

• In a small bowl, mix breadcrumbs, butter, and Parmesan cheese with your hands until well combined.
• Scatter over the Brussels sprouts.

Bake:

• Bake uncovered for 20-25 minutes until bubbly and golden brown.
• If breadcrumbs brown too quickly, cover lightly with foil at the 15-minute mark.
• Serve immediately.