Cut the tops of the ciabattini flat and halve horizontally.
Wash the tomatoes, remove the stem, cut in half lengthways and cut out the green and white stalk. Halve each half lengthways, remove the kernels and cut the quarters into pieces the size of a kernel of corn. Season the tomato pieces together with the strained corn kernels with salt and pepper and oregano. Cut the ham and cheese slices onto the ciabattini.
Remove the required leaves from the Roma salad, wash and place on a serving plate as a mini bowl.
Heat the olive oil in a large pan. Add the ciabattini with the cut surfaces facing down and roast until light brown. Put the ciabattini in an ovenproof bowl, cover with the ham and spread the corn-tomato mixture on top. Finish with the cheese and cut the optional olives in half and press onto the cheese. Bake with top heat until the cheese has melted. Put the finished crostini in the salad bowl, serve and enjoy.