In a small bowl, whisk together Greek yogurt, taco seasoning, and lime juice. Set aside. In a large skillet, brown chicken with onion powder, chili powder, cumin, garlic salt, and fennel seed. Crumble into grounds as the chicken browns. To serve, spread a spoonful of the yogurt mixture into the bottom of each romaine leaf. Top with chicken. Serve topped with avocado, tomatoes, cilantro, and scallions. .