Preheat oven at 200 ºC. Mix chickpeas, parsley, onion, garlic, crushed coriander seeds, roasted cumin powder, baking soda, lemon juice and salt in a bowl.
Grind in a mixer without water. Add 1 tbsp olive oil and grind to a coarse paste resembling bread crumbs. Heat sufficient oil in a deep non-stick pan.
Meanwhile, make the tahina sauce: mix mayonnaise, salt, lemon juice, pepper powder and onion. Chop mint leaves and add.
Add tahina and mix well. Add 2 tsps warm water and roasted cumin powder and mix well.
Slice the pita bread into two horizontally. On each piece place a few tomato slices, a few onion slices and mint leaves. Sprinkle salt and lightly toast them in the preheated oven for 3-4 minutes.
Shape the falafel mixture into round tikkis and deep fry in the hot oil till golden and crisp. Drain on absorbent paper. Remove the pita slices from the oven and arrange them on a serving platter.
Place a few fresh mint leaves on each, followed by a little tahina sauce, over this place a falafel, sprinkle little roasted cumin powder and serve with black olives and pickled gherkins.