Make the Creamy Chimichurri Sauce. Combine 3 tbsp. Greek yogurt, 3 tbsp. mayonnaise, 2 tablespoon minced shallot, 1 heaping tbsp. finely chopped parsley, 1 heaping tbsp. finely chopped cilantro, 1 tbsp. good olive oil, 2 tsp. red wine vinegar, 1 small garlic clove, and ½ tsp. Diamond Crystal kosher salt in a small bowl. Whisk to thoroughly combine, then taste and season with more salt if needed. Set aside.
Season the Chicken. Combine ½ tsp. garlic powder, ½ tsp. Diamond Crystal kosher salt, and ½ tsp. freshly cracked black pepper in a bowl and stir to evenly combine. Season the chicken breast all over with the seasoning, then set aside for about ten minutes at room temperature.
Cook the Chicken. Heat 2 tsp. avocado oil in a pan over medium heat. Cook the seasoned chicken breast for about 4-5 minutes per side, until browned and the internal temperature reaches 165 F. Transfer the chicken breast to a bowl and cover with foil. Let it rest for at least five minutes, then shred it in the bowl.
Cook the Jalapenos and Red Onions. In the same pan, add a couple of tablespoons of water and scrape the browned bits into the water. Add the sliced jalapenos and chopped onions and cook for 3-4 minutes until softened.
Mix in the Shredded Chicken. Transfer the chicken and any liquids back to the pan with the jalapenos and onions.
Add Cilantro. Mix the chicken into the jalapenos and onions, then mix in the finely chopped cilantro. Turn off the heat.
Build the Tacos. Layer the shredded romaine, sliced avocado, and shredded chicken into warmed flour tortillas. Top with crumbled feta cheese, and drizzle with creamy chimichurri sauce. Serve warm!