Step 1: Prepare the Chicken : Season chicken breasts with salt and pepper. Brush both sides with a little olive oil. Grill, bake, or pan-sear the chicken until fully cooked (internal temperature 165°F / 74°C). Brush generously with BBQ sauce during the last few minutes of cooking. Remove from heat, let rest for 5 minutes, then slice thinly.
Step 2: Make the Ranch Dressing : In a medium bowl, whisk together mayonnaise, sour cream, and buttermilk until smooth. Add lemon juice, garlic, parsley, chives, dill, salt, and pepper. Adjust thickness with more buttermilk if desired. Chill in the fridge while preparing the salad.
Step 3: Prepare Salad Ingredients : Rinse and chop lettuce. Halve cherry tomatoes and dice avocado. Drain and rinse black beans and corn. Thinly slice red onion.
Step 4: Assemble the Salad : Arrange lettuce in a large bowl or platter. Add black beans, corn, tomatoes, avocado, onion, and cheese. Top with sliced BBQ chicken. Sprinkle crushed tortilla chips (optional).
Step 5: Dress and Toss : Drizzle ranch dressing generously over the top. Toss lightly to coat or serve dressing on the side for dipping.