1 garlic clove
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 beans, drained and rinsed
1 (1-lb) seedless cucumber (usually plastic-wrapped), quartered lengthwise and cut
crosswise into 1/3-inch-thick slices
1 lb tomatoes (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
Mince garlic and mash to a paste with a pinch of salt using a large heavy knife. Whisk together garlic paste, lemon juice, salt, and pepper in a small bowl, then add oil in a slow stream, whisking until combined well. Let stand at room temperature 10 minutes.
Gently toss beans, cucumber, tomatoes, and parsley with half of dressing. Transfer to a platter or bowl and arrange tuna on top, breaking up any large pieces with a fork. Drizzle remaining dressing over tuna.