In a large, heavy-bottom pot over medium heat, add the oil. When the oil is hot, add the onion, garlic, thyme, bay leaf, carrot, and celery. Season with a pinch each of salt and pepper and allow the ingredients to cook, stirring occasionally, until the onion is soft and translucent, about 5 minutes. Add the cooked beans, diced tomatoes, and stock. Bring to a light boil, then reduce the heat to low and simmer for several minutes; the stock will thicken. Adjust the seasoning if necessary, remove the thyme sprigs and bay leaf, and serve hot. This stew can be refrigerated for three to four days or frozen for four to six months.