INGREDIENTS
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9 ounces canned tuna (preferably packed in oil), drained
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1/3 cup mayonnaise
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1/2 celery stalk, finely sliced
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1/4 small (about 2 tablespoons) red onion, finely chopped
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1/4 medium (about 2 inches long) cucumber, peeled, if desired, and finely diced
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Juice of 1/2 lemon
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Sea salt and freshly ground black pepper to taste
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1/2 baguette, cut crosswise into 2 portions, or 4 slices bread, such as sourdough or multigrain
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4 slices tomato (optional)
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2 slices cheese of your choice, such as Gruyère, Swiss, or white Cheddar