Pound chicken to 1/2-inch thickness. Heat oil in a large skillet. Sprinkle chicken with cumin, salt, and cayenne pepper. Place chicken and garlic in skillet and cook over medium-high heat, 2 minutes per side. Cook
Add tomatoes; cover and simmer until chicken is cooked through, 5 to 6 minutes. Transfer chicken to a serving platter; keep warm.
juices in skillet over high heat until thickened, about 2 minutes. Reduce heat to low; stir in sour cream, avocado, and cilantro until well blended. Pour over chicken.