Preheat oven to hot, 200 C. Lightly grease two oven trays.
Toss tomatoes, onion, oil, garlic, vinegar and spices on one tray. Spread into a single layer. Roast 25-30 mins, until tender.
Transfer vegetables and roasting juices to a large saucepan. Pour stock over. Bring to the boil on high. Reduce heat to low and simmer, uncovered, 30 mins.
Using a stick blender, process until smooth. Season to taste and add a pinch of sugar. Heat on low, stirring.
Meanwhile, arrange prosciutto in a single layer on remaining tray. Bake 4-5 mins, until crisp. Crumble into pieces.
Divide soup between serving bowls. Top with torn bocconcini, prosciutto and fresh basil leaves. Accompany with bread.