Step 1

Preheat oven to 425 degrees F (220 degrees C). Lightly grease a baking sheet.

Step 2

Combine Brussels sprouts and onion together in a bowl; add 3 tablespoons olive oil, salt, and pepper and gently toss to coat. Spread sprouts mixture onto the prepared baking sheet.

Step 3

Bake in the preheated oven until sprouts and onion are tender and caramelized, 25 to 30 minutes.

Step 4

Heat remaining 2 tablespoons olive oil in a small skillet over medium-high heat; saute shallot until tender, about 5 minutes. Add balsamic vinegar and cook until glaze is slightly reduced, about 5 minutes. Stir rosemary into glaze and pour over sprouts mixture.