Spread olive oil on each slice of bread, cut the mozzarella into slices, and place 2 mozzarella slices, 2 prosciutto ham slices, and 1 basil leaf on 2 of the slices of oiled bread. Add salt and pepper and close the sandwiches with the remaining bread slices.
Dip the sandwich in milk, flour and then beaten eggs, then fry in a pan for a few minutes on each side over a medium heat.
Cut 2 pieces of clingfilm and place a sandwich on each piece, then close the clingfilm and twist to form 2 balls. Let that chill for 10 minutes in the fridge. Meanwhile, make a cheese sauce by heating cream and grated parmesan over a low heat.
Take the sandwich balls out the fridge, remove the clingfilm, and place them in an oven dish and bake for 15 minutes at 180°C. Serve the croque-monsieurs straight from the oven with the parmesan cream sauce, and enjoy.