To prepare pickled onion, put apple cider vinegar, sugar, salt and one-fourth cup water in a bowl and mix well. Add onion, mix well and set aside for one hour.
To prepare Greek yogurt sauce, put Greek yogurt, lemon juice, garlic, parsley, mint leaves and salt in another bowl and mix well. Set aside.
Put chicken mince, feta cheese, mint leaves, parsley, red bell pepper, salt and crushed peppercorns in a third bowl and mix well.
Divide the chicken mixture into four equal portions and shape them into patties.
Break eggs in a shallow bowl and beat well. Spread the refined flour on a plate.
Heat sufficient oil in a pan.
Coat the chicken patties in the flour, dip in beaten eggs and again coat with the flour. Shake away excess flour.
Deep-fry these coated patties in hot oil till golden. Drain on absorbent paper.
Halve the burger buns.
Put some Greek yogurt sauce on the base half of each burger bun and place some spinach leaves on top. Put a fried patty, top with some pickled onion and cover with remaining buns.