Generously coat the inside of a 7-inch round springform pan with olive oil. Set a heat- and pressure-safe trivet in a 6- or 8-quart cooker. Add the water. Prepare an aluminum foil sling.
Mix the ricotta, egg, and nutmeg in a medium bowl until uniform.
Spread a few tablespoons of the marinara sauce in the springform pam. Break one lasagna noodle to make a layer in the pan over the sauce, then dollop an even layer of 1/4 cup of the ricotta mixture in tiny bits on top, followed by a sprinkled, even layer of 1/4 cup mozzarella. Make three more layers, using 1/3 cup marinara sauce, a broken lasagna noodle, 1/4 cup ricotta mixture, and 1/4 cup mozzarella for each. After each layer, press the noodle bits down slightly to help spread out the ricotta below. Finish up with half of the remaining marinara sauce, the last lasagna noodle, the remainder of the marinara sauce, and the Parmigiano-Reggiano in an even layer. Cover the springform pan tightly with aluminum foil. Use the sling to lower the pan onto the trivet in the pot; fold down the ends of the sling so they fit inside the pot. Lock the lid on the cooker.
Press Pressure cook (Manual) on High pressure for 20 minutes with the Keep Warm setting off.
Once the machine has finished cooking at pressure, turn it off and let the pressure in the pot return to normal naturally, about 20 minutes. Unlatch the lid and open the pot. Use the sling to lift the springform pan out of the pot. Uncover the pan and set aside for 10 to 15 minutes so the lasagna sets up. Run a small knife around the interior edges to loosen the casserole from the pan, then unlatch the springform pan, remove the outer ring, and cut the lasagna into quarters to serve.