Tuna Salad with Fennel, Cucumber and Tarragon

1 small shallot, finely chopped 1/4 cup white wine vinegar 3 tablespoons extra-virgin olive oil 1 1/2 teaspoons sugar 1 tablespoon chopped tarragon 1 small fennel bulb—cored and cut into 1/4-inch dice 1/2 English cucumber—peeled, seeded and diced Two 6-ounce cans olive oil–packed tuna, drained Kosher salt and freshly ground pepper Lettuce leaves or pita bread, for serving