For a smoky, flame-grilled flavour, grill the burger patties over an open fire. Lightly toast the pita pockets to prevent the bread from getting soggy.
Add the pink peppercorns, rosemary leaves, coriander seeds and salt to a mortar and pestle and grind them together.
Using your hands, combine the minced beef with the red onions, garlic, egg and spice mixture from the mortar and pestle.
Divide the mince into 12 parts and roll each piece into a ball, then form into small flat patties.
To help firm up the patties, drizzle with olive oil, put them on a plate, cover and place in the refrigerator until needed.
Make the feta pesto by adding the mint leaves, chilli flakes, lemon juice, olive oil and half the pine nuts to a food processor or use a stick blender and blend into a paste. Crumble the feta cheese and remaining pine nuts into the pesto and season with freshly ground black pepper.
Heat a griddle pan on a medium-high heat, brush the burger patties with oil and cook for 2 or 3 minutes on each side or to your liking. Use a spatula to lightly press down on them, making sure each burger is in full contact with the pan.
Cut off and discard one-third of each pita, open up the pockets and fill each one with baby lettuce leaves, two burgers patties and top with the delectable feta pesto.