Preheat oven to 375°F (190°C).

Slice each chicken breast into 3-4 equally thick pieces.

In a large bowl, combine chicken with onion, bell pepper, cherry tomatoes, za'atar, olive oil, parsley, salt, and pepper.

Cut four 12x12 inch pieces of parchment paper.

Place mounds of chicken and vegetable mixture a few inches off the center of each parchment square.

Arrange lemon slices on top of each mound.

Fold the parchment over the mixture.

Fold and crimp the edges of the parchment paper to create a tightly sealed half-moon shape.

Place the packets on a rimmed baking sheet.

Bake for 20-25 minutes, or until chicken is cooked through.

Transfer each packet directly to a serving plate.

Break open the packets at the table and serve.