Beat the egg in a bowl. Add parsley, basil, garlic, salt, pepper, nutmeg, romano (or parmigiano), mozzarella and ricotta, and stir to combine thoroughly.
Pour about 1/3 jar of marinara sauce in the bottom of your pressure cooker. Add the water and stir.
Fill the (uncooked) pasta shells with the cheese filling with a spatula or piping bag. Be sure not to leave air pockets. Place in one layer in the bottom of the pressure cooker.
Cover the filled shells with the rest of the tomato sauce, spreading it out to make sure all the pasta is covered. Bring up to pressure over medium-low heat (do not be tempted to turn up the heat, or the sauce may burn). Cook 12 minutes at pressure. Release the pressure (with the quick release valve if you have one, or by running cold water over the pot if you don't) and serve. Serve with more grated romano or parmigiano if desired.