Cut each chicken breast in half lengthwise to make 4 thin pieces.
Place the chicken pieces in a large resealable bag or bowl; add 3 teaspoons Southwest seasoning, 1/2 teaspoon garlic powder, and 2 tablespoons olive oil. Toss to coat and let marinate while you prep other ingredients (refrigerate if marinating longer than 10–15 minutes).
Cook 1 cup uncooked rice according to package directions, then keep warm.
Make the sauce: in a small bowl combine 1/2 cup Greek yogurt, 1/2 tablespoon lime juice, 1/2 tablespoon sriracha, and 1 minced garlic clove; stir until smooth and set aside.
In a medium bowl, combine drained corn, drained and rinsed black beans, chopped 1/4 medium red onion, 1/2 chopped red bell pepper, chopped avocado, juice of 1/2 lime, 1 tablespoon olive oil, and chopped cilantro; toss gently and set aside.
Heat a large skillet over medium-high heat. Add the marinated chicken and sear 4 minutes per side until browned. Reduce heat to medium-low, cover, and cook a few more minutes until chicken is cooked through (time will vary by thickness).
Slice the cooked chicken, divide rice among four bowls, top with sliced chicken and the bean-corn-vegetable mixture, and drizzle with the yogurt-sriracha sauce to serve.