Preheat oven to 425 degrees.
Line a baking pan with foil.
In a large bowl, toss the Brussels sprouts, olive oil, red onion, salt, pepper, and mustard seasoning.
Transfer to the foil lined pan and spread the veggies out evenly.
Allow the vegetables to roast for 25-30 minutes.
Remove the pan from the oven. Drizzle the balsamic vinegar over the veggies.