Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Trim the woody ends off the Brussels sprouts and slice them in half; dice the chicken breast into uniform 1-inch pieces.
In a large mixing bowl, combine the halved Brussels sprouts, chicken pieces, and diced pancetta.
Drizzle with olive oil and sprinkle with sea salt and black pepper, tossing thoroughly to ensure everything is evenly coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the Brussels sprouts are cut-side down for maximum caramelization.
Roast for 20 to 25 minutes, tossing halfway through, until the chicken is cooked through and the sprouts are tender and charred.
While the sheet pan roasts, combine the balsamic vinegar and honey in a small saucepan over medium-low heat.
Simmer the balsamic mixture for 3 to 5 minutes until it has reduced by half and coats the back of a spoon.
Remove the tray from the oven, drizzle the warm balsamic glaze over the top, and garnish with fresh thyme leaves before serving.