Preheat your oven to 425°F (220°C) and line a large baking sheet with parchment paper for easy cleanup.
Prepare the Brussels sprouts by trimming the stems and removing any discolored outer leaves. Cut them in half lengthwise—this creates more surface area for caramelization, which equals more flavor!
In a large mixing bowl, toss the halved Brussels sprouts with 2 tablespoons olive oil, salt, and pepper until evenly coated. Spread them in a single layer on one side of the prepared baking sheet.
Scatter the bacon pieces among the Brussels sprouts, distributing them evenly. The bacon fat will render and help the sprouts become extra crispy.
In the same mixing bowl (no need to wash it), toss the sliced red onion with the remaining 1 tablespoon olive oil, balsamic vinegar, and maple syrup. Spread the onion mixture on the other side of the baking sheet.
Roast in the preheated oven for 15 minutes, then remove the baking sheet and add the minced garlic and thyme evenly over everything. Stir to mix and flip the Brussels sprouts to ensure even browning.
Return to the oven and roast for an additional 10-15 minutes until the Brussels sprouts are deeply golden with crispy edges, the bacon is crisp, and the onions are caramelized and slightly sticky.
Combine everything on the baking sheet by gently tossing together the Brussels sprouts, bacon, and balsamic onions. Taste and adjust seasoning if needed.
Transfer to a serving dish and garnish with toasted nuts and Parmesan cheese if using. Serve immediately while hot and crispy.