Tuna and White Bean Salad

3 tablespoons fresh lemon juice

3/4 teaspoon salt

1/2 teaspoon black pepper

1/4 cup extra-virgin olive oil

1 (15- to 19-oz) can cannellini or other white bean s, drained and rinsed

1 (1-lb) seedless cucumber (usually plastic-wrapped), quartered lengthwise and cut crosswise into 1/3-inch-thick slices

1 lb tomato es (3 medium), cut into 1-inch pieces

1 1/2 cups loosely packed fresh flat-leaf parsley leaves

2 (6-oz) cans tuna packed in oil (preferably olive oil ), drained