Heat oven to 350ºF. Boil orzo according to package. Drain.

Prepare peppers by slicing off the tops and removing the seeds and membranes from the inside. If needed, cut a thin slice off the bottom of each pepper to help it stand up. Arrange peppers in a baking dish.

Heat the oil in a medium saucepan. Add the onion and cook 3-5 minutes, or until soft. Add the cooked orzo, sauce, pepperoni, seasonings, and 3/4 of the cheese. Divide the filling between the peppers. Pour about 1 cup of water into the bottom of the baking dish, around the outside of the peppers. Cover with foil.

Bake for 30 minutes, or until peppers soften. Top the peppers with remaining cheese and return to oven until cheese melts - about another 5 minutes.