In a stock pot, whisk the hot stock into the roux. In a large saute pan, heat the bacon fat. Add the vegetables. Season with salt and pepper. Saute until wilted, about 5 minutes. Stir the tomato puree into the vegetables and cook for about 5 minutes. Add the tomato/vegetable mixture to the stock/roux mixture. Add the bouquet garni and continue to simmer, skimming as needed. Season with salt and pepper. Simmer the sauce for about 45 minutes. Strain the sauce through a China cap.