Clean and husk corn, then prepare steamer. In a large pot, add 1-2 inches of water and bring to a simmer. Add corn to steamer basket and cover. Steam corn until done, about 10 minutes. When corn is cool enough to handle, cut corn from cob.
Wash the lettuce, tear into bite size pieces, and place on a platter. Add the corn, onion, tomato, cucumber, and avocado. Sprinkle with chopped cilantro. Dress with the lemon juice, olive oil, salt and pepper.