Make the dressing: In a small bowl, combine sour cream, minced cilantro, minced seed-removed chipotle pepper, lime juice, cumin, chili powder, and 1/4 teaspoon salt; mix until smooth and set aside.
Prepare the chicken: Season pounded chicken breasts with salt, pepper, and taco seasoning (or additional cumin and chili powder).
Cook the chicken: Heat a lightly greased large skillet over medium-high heat and cook the chicken 5–8 minutes per side until an instant-read thermometer reaches 160°F; transfer to a plate and let rest 5 minutes.
Deglaze and dice: Add 2 tablespoons chicken broth or water to the hot skillet and scrape up browned bits, then pour those drippings over the rested chicken. Dice the chicken into bite-sized pieces and set aside.
Cook onions and vegetables: Spray the skillet again if needed and cook the sliced red onion until just shy of your preferred doneness (skip this step if you want raw crunchy onion). Add the drained black beans and corn and cook until heated through.
Combine chicken and vegetables: Return the diced chicken to the skillet and stir to combine and warm through.
Toss the salad: In a large bowl, combine chopped romaine, the chicken–bean–corn mixture, chopped tomatoes, and the dressing; toss until everything is evenly coated.
Serve: Top with shredded cheddar and optional crushed tortilla chips, then serve immediately.