• Begin by preheating your oven to 425°F.
• In a large skillet over medium heat, crisp the diced bacon until all the fat has rendered. Remove the bacon with a slotted spoon and place it on a paper towel-lined plate.
• Retain about 1 tablespoon of the rendered bacon fat in the pan, discarding any excess (or save it for a different recipe). Add the butter to this fat.
• Introduce the diced onion along with a pinch of salt, and let it cook for about 15 minutes until it starts to caramelize.

Roasting the Brussels Sprouts:

• On a large rimmed baking sheet, spread out the halved Brussels sprouts. Drizzle them with olive oil and generously season with salt and pepper. Roast them in the oven for 15 minutes.
• After 15 minutes, give the Brussels sprouts a toss and continue roasting them for an additional 10 minutes until they are golden and tender.

Finishing the Dish:

• Once the onions are golden brown and caramelized, mix in the garlic, red pepper flakes, brown sugar, and balsamic vinegar. Stir the mixture frequently until the balsamic reduces and becomes thick and glossy. Taste and tweak seasoning with salt and pepper as needed.
• Take the Brussels sprouts from the oven and pour the balsamic onion mixture over them. Toss everything together until well coated, then stir in the crispy bacon.
• Transfer to a serving dish and enjoy while it's still warm!