Cut the woody end off the Brussels sprouts, remove the outer leaves and cut in half. Put in a large mixing bowl.
In a food processor combine the onion and garlic and pulse a few times to chop. Add the Vegenaise, balsamic vinegar, salt and pepper. Process until combined but not smooth. Pour the mixture over the Brussels sprouts, add the gluten-free bread crumbs and mix well. Pour the mixture into a shallow baking dish or cast iron skillet and cover with foil. Bake for 30 minutes, remove the foil, gently stir to bring the Brussels sprouts that were on the bottom to the top and return to the oven. Bake uncovered for 30 more minutes or until the sauce is browned and bubbly and the Brussels sprouts are tender when pierced with a paring knife.