In a heavy dutch oven heat the oil on about medium heat and whisk in the flour, keeping stirring and cooking until it turns brown color about like peanut butter. Add the celery, onion and peppers and garlic and sauté until tender. Whisk the stock back in and add in the tomato pesto (if you don’t have this use tomato paste). Simmer for about 3 hours and about 10 minutes before serving to add the chicken, sausage, and shrimp.