Wash and chop the romaine lettuce, then layer it in a large bowl.
Halve the cherry tomatoes and slice the cucumbers.
Soak the red onion in ice water for 5 minutes to mellow its bite, then drain well.
Shred the leftover grilled lemon herb chicken.
Gently toss the chicken with the prepared vegetables in the bowl.
Dice the avocado last and add it to the salad to help it stay firm.
For the dressing, whisk together olive oil, lemon juice, and Dijon mustard until smooth.
Season the dressing with oregano, salt, and pepper.
Drizzle the dressing over the salad and toss everything lightly to combine.
Ready to serve in under 15 minutes