Peel shrimp, and place in a bowl with 2 tbsp olive oil, minced garlic, juice of 1 lime plus salt and pepper to taste. Let marinate for 15 minutes.
In the meantime, slice avocados and mash them in a medium bowl, with 1 tbsp of olive oil, onion, salt and pepper to taste and set aside.
Grill shrimp, and once ready, warm each bun,(alternatively, it can be sautéed) top each side of Crustini buns with some avocado, shrimp, feta cheese, chili flakes, green onions and cilantro. Finish the dish by drizzling some chili oil on top.