Dissolve the yeast together with the sugar, salt and oil in 220ml lukewarm water. Mix the flour in a bowl with the yeast water and knead to a smooth dough. Cover and leave to stand for about an hour.
Meanwhile, wash the spring onions for the filling and cut into slices.
Finely dice the drained mozzarella and ham.
Heat the olive oil in a pan and saute the spring onions until softened. Peel and crush the garlic and add to the pan.
Tip the mixture into a bowl and allow to cool. Mix with the mozzarella and ham. Season with salt and pepper.
On a floured surface, shape the dough into logs, about 4cm in diameter, then cut into 2cm pieces.
Roll each piece of dough into a round and spoon on some filling.
Fold the dough over the filling to form a semicircle.
Press the dough edges with the prongs of a fork to seal.
Place the pasties on a baking tray lined with baking paper. Mix the egg yolk with 3-4 tbsp water and brush over the pasties. Sprinkle with the sesame seeds and bake in the oven at 180°C (160°C fan, gas 4) for 25 minutes until golden brown.