1 small red onion, peeled and very thinly sliced

1 tablespoon plus 1 teaspoon red wine vinegar or sherry vinegar

1 (6 1/2-ounce) can water-packed tuna, drained

1 (15-ounce) can cannellini beans or borlotti beans, drained through a strainer and rinsed

3 fresh sage leaves, slivered

2 tablespoons finely chopped flat-leaf parsley

Salt and freshly ground pepper

1 small or medium garlic clove, finely minced

1/2 teaspoon Dijon mustard

3 tablespoons extra virgin olive oil

1 tablespoon plain low-fat or nonfat yogurt (or omit and use 4 tablespoons olive oil)

1/2 Japanese cucumber, cut in half lengthwise and sliced, for garnish