To prepare soup, return Dutch oven to medium heat. Add oil to pan; swirl to coat. Melt butter in oil. Add sliced onion to pan; cook for 5 minutes, stirring occasionally. Partially cover, reduce heat to medium-low, and cook 15 minutes, stirring occasionally. Add salt and ground pepper; cook, uncovered, until deep golden brown (about 35 minutes), stirring frequently. Add reserved stock and chopped thyme; bring to a boil. Reduce heat, and simmer until reduced to 8 cups (about 50 minutes). Stir in chives.