Lightly score the chicken fillets before dipping them in the egg white and coating with the za'tar.
Wash the zucchinis, cut lengthways into strips about 5mm (1/4 in) thick, brush each strip with olive oil and sprinkle with sea salt.
Heat a well-oiled barbecue or chargrill pan and grill the chicken fillets for 6-8 minutes each side, depending on their thickness. Add the zucchini strips 5 minutes after you put the chicken on to cook, and grill for about 3 minutes each side. Add the lemon slices in the last 2 minutes.
Steam the spinach for 2 minutes and drain well.
Serve the chicken with the zucchini and lemon slices on top and the spinach on the side.