Boil the shrimp in a pot of water until fully cooked.
Remove the heads, peel the shrimp, and set them aside.
Chop the lettuce and spread it evenly on a large serving plate. Add the tomatoes, onion, and avocado slices on top.
To make the dressing: In a small bowl, mix the lemon juice, olive oil, garlic, salt, and black pepper well.
Pour the dressing over the ingredients on the plate.
Add the boiled shrimp, sprinkle the chopped cilantro on top, and then serve.