• Preheat the oven to 400 degrees F.
• Cut the fingerling potatoes in half lengthwise. Cut any large potatoes into quarters.
• Place the potatoes and Brussels sprouts into a medium bowl. Toss with the olive oil, rosemary, salt and pepper until coated.
• Transfer the potatoes and Brussels sprouts to a large baking sheet.
• Roast until the potatoes are tender and golden brown, about 30 minutes, stirring halfway through.
• Transfer to a serving bowl or platter, and drizzle with the balsamic glaze.

Homemade Balsamic Reduction:

• Pour the balsamic vinegar into a small saucepan and set over medium-high heat. Bring to a boil, then reduce heat to a simmer.
• Continue to simmer, stirring occasionally, until the vinegar reduces by at least half and coats the back of a spoon. Watch carefully so that the vinegar doesn’t burn.
• Remove from heat and allow to cool. The reduction can be used right away or store in the fridge in an airtight container.