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. Cocktail Party RecipesPlus: More Appetizer Recipes and Tips
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.
Don't mix this salad too far in advance, since the acid in the vinegar will discolor the herbs and wilt the leaves.