Prepare the marinara sauce, following the recipe in our Tomato Sauce 101 post, leaving it a bit looser than you would normally.
Boil the shells in well salted water until only three-quarters cooked.
While the shells are cooking, mix the ricotta, Parmesan, egg, parsley, salt and pepper together in a mixing bowl until you have a smooth cream. Taste and adjust for seasoning. Then fold in the cubed mozzarella.
When the shells are done, drain them and lay them out to cool on a backing rack or on a kitchen towel.
When the shells are cool enough to handle, fill them with the ricotta and mozzarella mixture. You can do this by pinching them top and bottom until they open slightly, then plop in a generous spoonful of the filling with a spoon. Repeat with all your shells.
Line the bottom of a baking dish (or individual baking dishes as pictured in this post) with a bit of the marinara sauce. Place the stuffed shells on top, open side up, then nap the shells with more marinara sauce. Top the shells with a generous sprinkling of grated Parmesan cheese and drizzled them all over with olive oil.
Bake in a hot (200C/400F) oven for 20-30 minutes, until the shells are cooked through and nicely browned on top.
Let the baked stuffed shells rest for 5 minutes or so before serving, with more marinara on the side for those who want some.