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Adjust rack to top position and preheat oven to 425 degrees. Wash and dry all produce.
Dice baguette into ½-inch pieces. Halve, peel, and thinly slice shallot. Thinly slice celery. Strip half the thyme leaves (all the leaves for 4 servings) from stems; mince leaves.
In a small bowl, combine half the stock concentrates (you’ll use the rest later) and ¼ cup water (½ cup for 4).