In a dutch oven (or large saucepan), melt the butter. Add the celery, carrots, onion, garlic, thyme and a generous pinch of salt and pepper. Cook over moderate heat, stirring occasionally, until the vegetables just start to soften, about 10 minutes. Sprinkle the flour over the vegetables and cook, stirring, until evenly coated and lightly browned, about 3 minutes.