Heat the oven to 425°F. Line a large rimmed baking sheet with parchment if desired, and preheat the baking sheet as the oven heats for better browning.

Toss the sweet potato cubes with 1 1/2 tablespoons olive oil, about 1/2 teaspoon kosher salt, and half of the black pepper. Spread them in a single layer on the baking sheet and roast for 15 minutes.

Toss the halved Brussels sprouts with the remaining olive oil, remaining salt, and pepper. Add them to the pan with the sweet potatoes, arranging as many Brussels sprouts cut-side down as possible.

Roast the vegetables for another 15–20 minutes, leaving the Brussels sprouts undisturbed for most of the cooking time. They are ready when the vegetables are browned and tender.

While the vegetables roast, combine the balsamic vinegar, honey, Dijon mustard, and minced garlic in a small saucepan. Simmer over medium-low heat for 3–5 minutes, stirring frequently, until slightly thickened, then remove from the heat before it reduces too far.

Transfer the roasted vegetables to a serving bowl or platter and drizzle with the warm glaze. Toss gently, then finish with toasted pecans, walnuts, or dried cranberries if using.