Preheat the oven to 200C/180C Fan/Gas 6. Line a baking tray with baking parchment.
Warm the oil in a non-stick frying pan over medium-high heat. Once hot, add the red onions and stir. Reduce the heat, cover with a lid and cook for 15 minutes. Add the vinegar and sugar, increase the heat slightly, and cook, uncovered, for a further 5 minutes. Set aside to cool completely.
Sift the flour, bicarbonate of soda and salt into a bowl. Using your fingertips, rub in the butter until it resembles breadcrumbs.
Reserve 1 tablespoon of the cooked onions, a small handful of cheese and a small amount of prosciutto for the topping. Add the remaining onions, prosciutto and cheese to the flour mixture. Add the basil and toss to incorporate.
In a jug mix the buttermilk with 25ml/1fl oz water. Make a well in the middle of the flour mixture and add three-quarters of the thinned buttermilk. Mix using a lightly floured hand or a table knife until it comes together into a rough but soft and sticky dough. Add a little more thinned buttermilk if the dough feels dry and won’t come together.
Turn the dough out onto a lightly floured work surface, roughly shape and transfer to the prepared baking trays. Flatten the loaf to about 4cm/1½in thick. Dust with a little flour and score the top using a sharp knife. Cover with the reserved onions, prosciutto and cheese.
Bake for 35-40 minutes or until golden-brown and the loaf sounds hollow when tapped on the base. Cover the top with foil if it’s starting to brown too much.
Transfer to a wire rack, brush liberally with melted butter and allow to cool before eating.