Mix all taco seasoning ingredients in a small bowl. Set aside half for the dressing.
If marinating overnight: coat the chicken with half the taco seasoning and a light drizzle of oil. Cover and refrigerate.
Cook the chicken: bake on a sheet pan at 400°F for 20–22 minutes or cook on the stovetop over medium-high heat until the internal temperature reaches 165°F.
Rest chicken for 5 minutes, then slice thin and shred into strips to keep it juicy.
Stir the remaining taco seasoning into the mayonnaise to make the dressing; taste and adjust salt as needed.
In a large bowl, combine shredded chicken, black beans, roasted red peppers, pickled jalapeños, and red onion. Toss to mix.
Portion the romaine (or romaine and cabbage mix) into the bottoms of 4 glass meal prep containers. Top with the chicken mixture.
Pack the dressing separately in small containers. Add optional corn and/or shredded cheddar if using.
Let components cool briefly, seal, label with today’s date, and refrigerate. Good for up to 5 days.