In a small bowl, whisk together all dressing ingredients until emulsified.
Divide the dressing evenly among 4 wide-mouth mason jars (about 2 tbsp each).
Layer the chickpeas and tuna on top of the dressing (this prevents sogginess).
Add the cucumber, tomatoes, red onion, and olives as the next layer.
Top with the feta cheese, then tightly pack the mixed greens into the top of the jar.
Seal with a lid and store in the fridge for up to 3 days.
To eat, shake the jar vigorously to distribute the dressing, then tip onto a plate or eat straight from the jar.