Preheat the oven to 180°C or set it to grill.
Remove the stems from the mushrooms, then cut these into tiny bits.
Cut the olives into small pieces too.
Mix the leftover Bolognese sauce with the mushroom and olive pieces - you should get a thick filling.
Fill the mushrooms with the filling, then put them on a lightly oiled baking tray.
Cut the mozzarella into thin slices and put one slice on top of each mushrooms.
Put the tray in the oven and bake/grill the mushrooms for 10-12 minutes, until the cheese is golden and starts melting.
Serve warm.