This recipe may be doubled or tripled to feed large crowds.
Preheat your oven to 400°F. Line a sheet pan (or roasting pan) with aluminum foil, or spray lightly with non-stick cooking spray.
Use a knife to slice the dried, woody ends of the Brussels sprouts off and discard, then the ends. Remove and discard any outer leaves that are brown or dried out.
Slice each sprout in half lengthwise (through the stem end), and place them onto your prepared roasting pan.
Use your forefinger and thumb and grip the top of the rosemary stem. Use the fingers of your opposite hand to remove the rosemary leaves by sliding your fingers, pinched around the stem, in a downward direction.
Finely chop the rosemary leaves.
Use a microplane to grate the garlic, or mince it with a knife.
Add the rosemary leaves, garlic, salt, pepper, balsamic vinegar, extra virgin olive oil, and the dijon to a medium size mixing bowl.
Whisk everything together until you have a smooth, emulsified dressing.
Pour the Balsamic-Rosemary vinaigrette over the Brussels sprouts, and toss to coat the them in the vinaigrette. Arrange the sprouts so they are all lying on their flat, cut sides.
Put the pan into your preheated oven and roast for 30-35 minutes. The sprouts should be a deep caramel color and the leaves should be crisp.
Remove the pan from the oven once the sprouts have finished roasting, and use a spatula to transfer them to a serving dish or your plates. Enjoy!