Preheat the oven at 180 degrees C
Halve the peppers and remove the seeds and pith
Lay on a baking tray and brush with olive oil
Set aside
Slice the cherry tomatoes
Slice the black olives
In a bowl mix the onions, tomatoes, olives and basil
Season with black pepper
Divide the mixture evenly among the peppers
Place in the oven for 20 minutes
Slice the mozzarella
Place slices of mozzarella on top of each of the pepper halves
Return to the oven and bake for a further 20 minutes until the peppers are soft and the cheese is melted and browning