For the filling
Put the flour, salt (if using), yeast, oil and 300ml lukewarm water in a bowl and mix until soft. Knead into a ball (try not to add any extra flour) – it will be sticky but the flour will absorb some moisture. Return to the bowl, cover and leave somewhere warm.
Meanwhile, make the filling. Heat the oil in a large non-stick pan, then stir-fry the peppers for about 1 min until they start to soften. Add the aubergine and garlic and continue to cook over a medium heat for 8-10 mins, gently pressing the veg with a wooden spoon until it breaks down a little. If it doesn’t, fry, covered, for a few extra mins.
Stir in the tomato purée, vinegar and 2 tbsp water. When the veg is soft, remove the pan from the heat and stir through the basil.
Heat the oven to 220C/200C fan/gas 7. Quarter the risen dough and roll each piece out to a 20cm circle on a lightly floured surface. Spoon a quarter of the filling over one side, scatter over a quarter of the olives, top with a quarter of the cheese, and brush the edges with the milk or beaten egg. Fold the dough over the filling and pinch the edges together at the side, a bit like making a Cornish pasty. Lift onto a lightly oiled baking sheet and brush with more milk or beaten egg. Repeat with the remaining dough and filling to make four calzones, then bake for 15-20 mins until golden. Leave to cool slightly and serve at room temperature.
