In a large mixing bowl, toss your halved Brussels sprouts with the cranberries, oil, and salt and pepper. Once evenly coated, spread onto a baking sheet in a single layer.
Roast in a preheated 400°F oven for 20-25 minutes, on the middle rack, until Brussels sprouts are crispy and browning on the edges. Stir them halfway through cooking.
Meanwhile, add the balsamic vinegar and sugar to a small saucepan and whisk to combine. Bring to a boil, then reduce heat to low and let simmer for about 15-20 minutes, until reduced by half.
When Brussels sprout mixture is done cooking, transfer to a serving dish. Drizzle half the balsamic glaze over the Brussels sprouts and cranberries and toss gently to coat. Add more of the glaze to taste (you may not need all of it) and continue to toss until evenly coated.