Preheat the oven to 375 degrees.
Place the brussels sprout halves and sliced onion on the baking sheet. Drizzle the olive oil on top then season with sea salt and freshly cracked pepper, to taste. Toss to coat evenly.
Roast in the oven for 20-25 minutes, stirring once halfway through cooking time, until golden and tender.
Make the sauce while they are roasting by combining the balsamic vinegar, honey, and dijon mustard in a bowl; whisk until well combined. Set aside.
Drizzle sauce, to taste, on the roasted veggies and toss to coat evenly. Serve immediately. Enjoy.