Drain the peppers and set them upside down on paper towels for 10 minutes. Any brine left inside will thin the filling and leak onto the platter.
Beat the cream cheese, ricotta, and Parmesan together until completely smooth and light. Room-temperature cream cheese is essential; cold cheese stays lumpy and will clog a piping bag.
Fold in the basil, parsley, garlic, lemon zest, prosciutto if using, and pepper.
Transfer the filling to a piping bag or a zip-top bag with the corner snipped off, and fill each pepper generously so the filling domes slightly above the rim.
Chill 30 minutes to firm up, then serve at room temperature.