For the Creamy Chimichurri Sauce: In a bowl, combine mayonnaise, Greek yogurt, minced shallot, chopped parsley, chopped cilantro, garlic, olive oil, red wine vinegar, and kosher salt. Stir until smooth and well combined. Set aside.,For the Shredded Chicken Filling: Heat avocado oil in a skillet over medium-high heat. Season the chicken breast with garlic powder, kosher salt, and black pepper. Cook the chicken for 5-6 minutes on each side, or until fully cooked and no longer pink in the center. Remove from heat and shred the chicken with two forks.,Add the sliced jalapeno, chopped red onion, and cilantro to the shredded chicken, mixing well to combine.,To Assemble: Warm the flour tortillas in a dry skillet or microwave. Place a few spoonfuls of the shredded chicken mixture on each tortilla.,Top the chicken with shredded romaine lettuce, sliced red onion, avocado slices, and crumbled feta cheese.,Drizzle with the creamy chimichurri sauce and serve immediately.