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Wash and dry all produce. In a medium pot, combine 4 cups water (7 cups for 4 servings) and half the stock concentrates. Bring to a boil, then reduce to a low simmer. Meanwhile, halve tomatoes. Zest and quarter lemon (quarter both lemons for 4). Pick basil leaves from stems; discard stems and roughly chop leaves. Mince or grate garlic.