Prep the chicken. If using rotisserie chicken, remove skin and bones, then chop or shred.
If cooking from scratch, season chicken breasts with salt and pepper, sear in a skillet with a little oil for 5–6 minutes per side, and let rest before slicing.
Mix the dressing. In a small bowl or jar, whisk olive oil, lemon juice, Dijon, honey, garlic, salt, and pepper until smooth and slightly thickened. Taste and adjust lemon or salt as needed.
Prep the produce. Halve the tomatoes, dice the cucumber, slice the onion thin, and chop the herbs. Cut the avocados last to keep them from browning.
Build the base. Add mixed greens to a large bowl.
Drizzle with a tablespoon of dressing and toss lightly so the greens are just coated.
Add chicken and veggies. Top the greens with chicken, tomatoes, cucumber, red onion, and herbs. If using feta and nuts or seeds, add them now.
Finish with avocado. Gently fold in the diced avocado. Be careful not to mash it—keep those chunks intact.
Dress and serve. Pour over the remaining dressing and toss lightly to combine.
Season with a pinch more salt and pepper if needed, and serve right away.