1. Preheat the oven to 450°F.
2. In a bowl, place together mushrooms, cipolline and pearl onions.
3. Spoon basting oil over mushrooms.
4. Season the onions mixture with salt and pepper, and mix well.
5. In a parchment lined baking tray, spread the onion and mushrooms in single layer.
6. Bake the vegetables in oven for 15 minutes.
7. Reduce the oven temperature to 350°F, and bake for 30 minutes more or until browned evenly.
8.Spoon the onions and mushroom on serving plate.
9. Drizzle with finishing sauce on top and serve hot.