In a small bowl, whisk together olive oil, lemon juice, lemon zest, herbs, capers, salt, and pepper.
In a large bowl, toss together chicken, cucumber, and green onions. Add dressing to chicken mixture and stir to evenly coat filling.
Halve croissants lengthwise. Fill bottoms with salad, dividing evenly, and set tops on salad.
Note: Nutritional analysis is per mini sandwich.