Cut the chicken breast into small, bite-sized pieces to ensure even cooking.
In a small bowl, combine the ground cumin, chili powder, sea salt, and black pepper.
Toss the chicken pieces with the spice blend until every piece is thoroughly coated.
Heat the olive oil in a large skillet over medium-high heat and add the seasoned chicken.
Sauté the chicken for 6-8 minutes, stirring occasionally, until cooked through and slightly browned.
While the chicken cooks, finely dice the bell pepper and red onion for the topping.
In a small blender or food processor, combine the avocado, Greek yogurt, and lime juice until the mixture is completely smooth and creamy.
Wash and carefully dry the butter lettuce leaves, then lay them out on a serving plate as taco shells.
Divide the sautéed chicken evenly among the lettuce leaves and top with the diced bell pepper and red onion.
Drizzle the avocado crema over each taco and garnish with freshly chopped cilantro before serving.