Combine the chicken, 2 cloves minced garlic, 1 tablespoon olive oil, and 2 tablespoons taco seasoning in a large bowl or a zip-top bag; toss to coat evenly and refrigerate for 15–30 minutes, or up to 24 hours.
Preheat a grill or skillet over medium-high heat and lightly oil the surface if needed.
Remove the chicken from the marinade and discard any remaining marinade. Grill or cook the chicken until no longer pink in the center and an instant-read thermometer reads 165°F (about 9–10 minutes per side for thicker pieces); transfer to a cutting board and let rest 5 minutes, then slice or shred.
While the chicken cooks, make the cilantro sauce: place 1/2 cup Greek yogurt, 1/2 cup loosely packed cilantro, 2 tablespoons olive oil, 1 clove minced garlic, juice of 1/2 lime, jalapeno (if using), and a pinch of salt into a food processor or blender; blend until smooth and creamy.
Prepare the toppings: dice the avocado, tomato, and 1/4 cup onion; set aside.
Assemble lettuce wraps by placing a portion of sliced or shredded chicken onto each Romaine leaf, then top with tomato, onion, avocado, and a drizzle of the cilantro sauce.