Simple winter vegetables—leeks, celery, spinach, and potatoes—are cooked together for an elegant flavor. Half puréed, the texture is both smooth and chunky.
In a large soup pot, cook the bacon over low heat, until fat is rendered, 5 minutes. Remove the bacon with a slotted spoon, and discard.
Add the butter to the pot. When it has melted, add the leeks, onions, and celery. Cook over low heat until wilted, 15 minutes. Season with the tarragon, thyme, and salt and pepper. Stir well.