Esca, which means "bait," is Mario Batali's Italian seafood restaurant in New York City. He uses mackerel for the bruschetta, but tuna is a fine substitute.
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In a bowl, combine the olive oil, vinegar, basil, garlic, crushed red pepper and minced rosemary. Season the dressing with salt and pepper.
In a large bowl, combine the beans with the mackerel and onion and toss gently. Add the dressing and toss again.
Mound 1/4 cup of salad on each toast, garnish with rosemary sprigs and serve.