Slice eggplant into ½ inch slices. Lay on a baking sheet (you will probably need to use 2 sheets) and salt liberally on both sides, allow to sit for about 30 minutes (use kosher salt here if you can. The coarser size will help keep the dish from getting to salty. If you don't have kosher salt, be more careful with the amount you use to avoid making the eggplant to salty)
After eggplant has been sitting for 30 minutes, blot with paper towel to remove moisture and excess salt, brush with olive oil and sprinkle with pepper (no need to salt since you already did)
Roast in the oven for 15-20 minutes or until golden. Remove from oven and adjust temperature to 375°F.
While eggplant is sweating and roasting, start the sauce.
Heat 1 tablespoon olive oil in a dutch oven or heavy pot over medium heat. When heated, add onion and cook, stirring occasionally, for a few minutes until soft. Add the garlic and cook a few more minutes, until fragrant. Increase heat to medium high and add the chicken sausage, salt, pepper, oregano and red pepper flakes. Cook, breaking up the meat into smaller pieces, until meat is no longer pink. If you have excess moisture in the pot, continue cooking for a few more minutes to evaporate the extra liquid, stirring occasionally.
Add the crushed tomatoes and bring to a simmer, then reduce heat to low and simmer for about 5 more minutes. Remove from heat and set aside.
For the ricotta mixture, combine the ricotta, Parmesan, egg, basil, salt and pepper.
To assemble, spread about 1/2 cup of sauce on the bottom of a 9x13 baking dish. Follow with a layer of eggplant slices. Spread 1/3 of the ricotta mixture on top of the eggplant. Sprinkle 1/3 of the mozzarella cheese on top of the ricotta mixture. Top with about 1 1/2 cups of the sauce. Repeat 1 layer of eggplant, ricotta, mozzarella and the remaining sauce. Top with last slices of eggplant, remaining ricotta, remaining mozzarella and sprinkle about 1/2 an ounce of finely grated Parmesan.
Tightly cover with foil that has been sprayed with cooking spray to avoid sticking. Bake in oven for about 15 minutes. Remove foil and continue baking for 25-30 more minutes, until top is bubbling and golden. If the top isn't browning the way you want it to, finish under the broiler for 1-2 minutes to get it nicely golden.