Place the sesame, nigella, cumin seeds and hazelnuts in a dry frying pan over a medium heat. Cook for 2-4 minutes, stirring until fragrant and lightly toasted.
Place the seeds in a pestle and mortar. Add the chilli, garlic and salt and grind until smooth.
Place the tahini, 50ml/2fl oz of the olive oil, the lemon juice, sumac and mint in a bowl. Add the spice mixture and stir until well combined. If the sauce is too thick, loosen it with a little water.
Heat the remaining oil and the butter in a large heavy-based frying pan. Add the prawns and fry for 4-5 minutes, or until pink all over.
Add the sauce and cook the prawns for a further two minutes.
Serve the prawns on a flatbread with a lemon wedge and pickled chillies.