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Drain and rinse the cannellini beans. Mash with a fork or puree them in a food processor for a minute or two until smooth.
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Combine the white bean puree with the tuna, mustard, lemon juice, olive oil, celery, pickle, salt, and pepper in a medium mixing bowl. Mix well.
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Turn the broiler on high.
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Spread a few spoonful’s of the tuna salad on each slice of bread. Cover with a slice of cheese, place on a baking sheet, and place under the broiler for 2 to 5 minutes until the cheese melts. Serve immediately.