Heat 2 tsp olive oil in a large heavy frying pan over medium-high heat.
Add 1 tsp minced garlic and the 4 oz can diced green chiles; cook, stirring, about 1 minute.
Stir in 1.5 tsp ground cumin and 1/4 tsp ground chipotle or cayenne and cook about 1 minute more to bloom the spices.
Add 2 lb ground turkey and 1 tsp salt; break the meat apart with a turner and cook until the liquid has evaporated and the turkey begins to lightly brown, stirring occasionally. Do not rush browning.
While the turkey cooks, thinly slice 1/2 cup green onions and finely chop 1 cup cilantro; wash and dry the cilantro first.
Peel and dice avocado (if using), place in a bowl and toss with 2 T fresh lime juice; stir in chopped tomato and chopped cilantro and season with salt to taste.
Cut off the root end of the iceberg lettuce, remove the core and discard tough outer leaves; cut remaining head into quarters to form cups for filling.
When the turkey is lightly browned, add the sliced green onions and cook 1–2 minutes more.
Remove the pan from heat and stir in the cup of chopped cilantro and 2 T lime juice; adjust seasoning with salt if needed.
Spoon turkey mixture into lettuce cups and top with the avocado–tomato–cilantro mixture as desired.