Make the dressing: In a food processor or blender, combine the avocado (for dressing), 2 garlic cloves, 1/4 cup cilantro, 1/4 cup Greek yogurt, 3 Tbsp avocado oil (or olive oil), 1 Tbsp lime juice, 1/4 tsp cumin, and 1/4 tsp each salt and pepper. Process until smooth.
Thin the dressing: If needed, add 1/4 to 1/2 cup water, a few tablespoons at a time, until the dressing reaches your desired consistency. Scrape down the sides and blend again.
Marinate the chicken: Whisk together 2 Tbsp lime juice, 1 Tbsp olive oil, and 1 Tbsp taco seasoning. Season with a little salt and pepper. Toss the 2–3 chicken breasts in the mixture and let marinate for 30 minutes or up to 12 hours in the refrigerator.
Cook the chicken: Preheat the oven to 400°F (200°C). Place the marinated chicken on a baking sheet and bake 15–20 minutes, or until an instant-read thermometer registers 165°F (74°C). Alternatively, grill 4–5 minutes per side on a hot grill or grill pan until cooked through.
Rest and slice chicken: Let the cooked chicken rest 5 minutes, then slice or dice it into bite-sized pieces.
Assemble the salad: In a large bowl, combine 8 cups chopped romaine, 1 cup halved grape tomatoes, 1 cup drained black beans, 1 cup corn, 1 diced ripe avocado (for salad), 1/2 thinly sliced red onion, 1/3 cup chopped cilantro, 1/3 cup crumbled cotija cheese, and the diced chicken.
Toss and serve: Drizzle the creamy avocado dressing over the salad, toss to combine, and serve immediately.