INGREDIENTS
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2 celery stalks, finely chopped
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 1/4 small red onion, finely chopped
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1 5-ounce can water-packed tuna, drained
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 1/4 cup mayonnaise
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2 tablespoons capers, drained, finely chopped
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2 tablespoons coarsely chopped fresh parsley
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1 teaspoon Dijon mustard
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1 tablespoon fresh lemon juice, plus more if needed
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Kosher salt, freshly ground pepper