Avocado Lime Salad

3 tablespoons fresh lime juice (1 large lime)
1/2 teaspoon salt, plus
1/8 teaspoon salt
4 small tomatoes, cut in 3/4-inch pieces (2 1/2 cups)
2 medium ripe avocados, cut into 3/4-inch dice (1 1/2 cups)
3 scallions, thinly sliced (white and green parts)
1 large garlic clove, minced
1 tablespoon extra virgin olive oil
1/4 teaspoon ground cumin
1 head boston lettuce, torn into large pieces (8 cups)
1 medium cucumber, peeled, quartered lengthwise, seeded and thinly sliced diagonally (1 cup)
2 tablespoons coarsely chopped fresh cilantro

1 In a medium glass or ceramic bowl, whisk together 2 tablespoons lime juice and 1/2 teaspoon salt.
2 Add tomatoes, avocados, scallions and garlic.
3 Using rubber spatula, gently fold to combine.
4 Season to taste with pepper.
5 In large bowl, whisk together olive oil, cumin, remaining tablespoon lime juice and 1/8 teaspoon salt.
6 Add lettuce and toss.
7 Season to taste with pepper.
8 Divide lettuce among plates.
9 Top each with mound of avocado mixture.
10 Top with cucumber slices.
11 sprinkle with cilantro and serve.