Pat the chicken breasts dry and rub 1 tablespoon taco seasoning evenly over both sides.
Heat 1 tablespoon olive oil in a skillet over medium-high heat. Add the seasoned chicken and cook until done, about 6–7 minutes per side, until no longer pink and juices run clear.
Transfer the chicken to a cutting board and let rest for 5 minutes, then slice into thin strips.
Meanwhile, make the cilantro-lime dressing: in a food processor combine 1 cup chopped cilantro, 1 chopped garlic clove, 3 tablespoons olive oil, 2 tablespoons lime juice, 3/4 teaspoon kosher salt, and 1/4 teaspoon crushed red pepper flakes. Blend until smooth.
Add 1/2 cup sour cream to the processor and blend until the dressing is fully incorporated and creamy.
In a large bowl toss 4 cups mixed greens with 1 cup corn, 1 cup drained black beans, 1 cup halved cherry tomatoes, 1/2 cup finely chopped red onion, and 1/2 cup diced red bell pepper.
Drizzle about half the cilantro dressing over the salad and toss to combine.
Divide the salad among 4 plates, top each with sliced chicken and avocado slices, drizzle with remaining dressing, and garnish with tortilla chips and additional cilantro if desired.