Preheat oven to 375 degrees and get 9x13 baking dish.
Slice eggplant into 1/2" rounds and season with salt on both sides then place on a cooling rack for 20 minutes to let it release moisture.
Pat the eggplant dry with paper towels well.
Add the olive oil to a large pan on medium heat and cook the onions for 3-4 minutes until translucent then add in the garlic and cook an additional 30 seconds.
Add the marinara sauce and mix well.
Mix together the ricotta, egg and parsley.
Add a bit of marinara sauce to the bottom of your baking dish then layer eggplant, ricotta mixture, marinara sauce and mozzarella cheese twice.
Top with the Parmesan cheese (save a bit for garnish) and bake, covered with foil, for 30-40 minutes.