In a saucepan, over medium heat, combine the reduction, onions, carrot, celery, bay leaf, and garlic cloves. Bring the liquid to a boil and reduce to a simmer. Cook the liquid until it reduces by half, about 20 to 30 minutes. The sauce will become a gloss. Remove the pan from the heat and strain. Set the liquid aside. In a large saute pan, heat the olive oil. When the oil is hot, add the spinach in batches and saute for about 2 to 3 minutes or until the spinach is wilted. Add the garlic slices. Season with salt and pepper. Stir the veal gloss into the pan and bring to a simmer. Stir in the truffle pieces and truffle butter. Serve the sauce warm.