Whisk the red wine vinegar (or lemon juice), olive oil, garlic powder, thyme, oregano, salt, and pepper in a small bowl until well combined.
Place the chicken breasts in a shallow dish or plastic zip-top bag, add the marinade, and toss to coat. Refrigerate for at least 1 hour, or up to 12 hours for extra flavor.
While the chicken marinates, prepare the salad ingredients. Chop the lettuce, cucumber, and onion, and grill the corn if it isn’t pre-cooked.
Heat a grill or grill pan to medium-high heat. Remove the chicken from the marinade (discard the leftover marinade) and grill for approximately 5 minutes per side, or until it reaches an internal temperature of 165°F. Let rest for 5 minutes before slicing.
To assemble the salad, divide the romaine lettuce between two bowls. Top with cucumber, cherry tomatoes, red onion, grilled corn, sweet potatoes, and sliced avocado.
Thinly slice the grilled chicken and place it on top of the salad. Finish with crumbles of feta (if using) and drizzle generously with honey mustard dressing.