Preheat oven to 140°C. Arrange the tomatoes in a single layer on a baking tray. Brush with oil and season with salt and pepper.
Roast for 1 hour 20 minutes until softened, but still just holding their shape. Combine the mayonnaise with the garlic and lemon juice and season with salt and pepper. Refrigerate.
Meanwhile, make the fritters. Slice the kernels off the corn cobs. Combine the flour and sugar in a large bowl. Whisk the egg and buttermilk together in a jug, pour onto the flour and whisk until smooth. Stir in the corn and season well with salt and pepper.
Heat 2 tbsp of the oil in a large frying pan over medium heat. Spoon the mixture one tbsp at a time into the pan and cook for about 2 minutes on one side.
Turn over and cook until golden. Keep warm while you cook the rest, adding more oil as required.
In a separate frying pan, fry the prosciutto in 1 tbsp of oil until crispy.
Serve the fritters accompanied by the roasted tomatoes, prosciutto and garlic mayonnaise.