Grate half of the cucumber. Squeeze out the excess liquid and add the Greek yoghurt and peeled and finely chopped garlic cloves. Mix well and leave to one side - this is your tzatziki.
Put the plain flour, corn flour, oregano and a pinch of salt and pepper on a large plate. Mix.
Crack the egg into a bowl. Beat.
Put the chicken breasts between two pieces of baking parchment and hit with a rolling pin until 1-1.5cm thick.
One at a time, dip the chicken breasts into the egg then the flour, ensuring that all sides are coated evenly.
Heat the oil in a large frying pan. Add the chicken breast and cook for 4-5 minutes on each side until golden brown and cooked through. Meanwhile, put the pittas in the toaster until warm.
Cut the tomatoes in half and slice the rest of the cucumber. Slice the pittas in half and spread with the tzatziki.
Cut the chicken into 1 inch slices and distribute between the pittas. Top with cucumber and tomato.