Heat the milk in a heatproof bowl in the microwave or in a pan over a low heat until lukewarm. Add the yeast and sugar, then leave to stand for 15 mins until frothy. Tip the egg yolks, flour, butter and ½ tsp salt into the bowl of a stand mixer fitted with a dough hook, and add the warm milk mixture. Knead for 10 mins – the dough should be tacky. Or, knead by hand for about 15 mins.
Cover and leave to prove for at least 1-2 hrs until doubled in size, or chill overnight. If you’re proving at room temperature, chill the dough for 30 mins after it has doubled to make it easier to work with.
Meanwhile, prepare the filling. Combine the olives, both the cheeses and the parsley, then set aside. Roll the dough out on a lightly floured work surface into a large rectangle (about 46 x 30cm).
Spread the pepper paste all over the dough. With a long side facing you, scatter half the filling over the half of the rectangle that’s closest to you, then fold the other half over that to enclose the filling. Gently roll out the dough again to help the filling stick to the dough, then cut the rectangle vertically into 20 x 2½cm strips. Working with one strip at a time, fold the strip loosely in half, then holding it at both ends, twist, then gently pull the loose ends through the loop.
Heat the oven to 220C/200C fan/gas 8 and line a baking sheet with baking parchment. Lay the simit on the sheet and brush with the beaten egg. Sprinkle over the sesame and nigella seeds, then leave to rest on the baking sheet for about 20 mins.
Bake for 25-30 mins, or until golden brown. Leave to cool slightly on the sheet before serving.