Preheat oven to 400°, on a nonstick sheet pan place red onion, Brussels sprouts, and celery. Toss with olive oil salt and pepper. Roast for 25 minutes.
Keep your oven on at 400°. In a mixing bowl combine your roasted vegetables with all remaining ingredients. Mix well.
Transfer dressing mixture an oven safe baking dish. Bake for 25-30 minutes until the top is golden brown.