Make the vinaigrette:
In a small jar, lemon juice, Dijon mustard, oregano, olive oil, salt, and pepper. Secure the lid and shake until fully emulsified.

Add vinaigrette to the jar:
Pour 1 to 4 tablespoons of vinaigrette into the bottom of each mason jar.

Add the chicken (optional):
Layer in the chopped, cooked chicken as the first solid ingredient.

Add vegetables and toppings:
Continue layering with cucumbers, feta cheese, kalamata olives, red onions, tomatoes, and finally, salad greens.

Seal and store:
Screw on the lid tightly. Store in the fridge for up to 3 days (if using chicken) or 5 days (without chicken).

To serve:
When ready to eat, shake gently or pour into a bowl and toss to coat with dressing.