1. Optional — salt the eggplant (reduces bitterness & moisture): Arrange slices in a single layer on a rack or layered between paper towels. Sprinkle both sides lightly with salt and let sit 20–30 minutes. Rinse and pat thoroughly dry. (Skip this step for young eggplant or if short on time.)
2. Preheat oven: Preheat to 400°F (200°C). Line baking sheets with parchment or lightly oil.
3. Roast or pan‑sear the eggplant slices: Brush both sides of eggplant slices with olive oil, season with pepper (and a little salt if you rinsed earlier). Roast on a single layer for 12–15 minutes, flipping halfway, until golden and tender. Alternatively, pan‑sear slices in batches over medium‑high heat 2–3 minutes per side until browned. Reduce oven to 375°F (190°C) after this step if roasting.
4. Make the tomato‑vegetable sauce: In a large skillet, heat 1 tablespoon olive oil over medium heat. Sauté onion 4–5 minutes until translucent. Add garlic, zucchini, mushrooms, and bell pepper (if using) and cook 4–6 minutes until softened and any mushroom liquid mostly evaporated. Stir in tomato paste and cook 1 minute. Add crushed tomatoes, oregano, basil, red pepper flakes (if using), salt, and a splash of water or broth if the sauce is very thick. Simmer 10–15 minutes to meld flavors; stir in balsamic vinegar if using. Taste and adjust seasoning.
5. Prepare the ricotta mixture: In a bowl, combine ricotta, egg (if using), 1/2 cup grated Parmesan, chopped parsley/basil, a pinch of salt, and a few cracks of black pepper. Fold in the fresh spinach (if using fresh, briefly chop; if using frozen, be sure it’s fully thawed and squeezed dry).
6. Assemble the lasagna: Spread a thin layer (about 1 cup) of sauce on the bottom of a 9×13‑inch (or similar) baking dish. Layer as follows: a single layer of lasagna noodles (or a layer of roasted eggplant for noodle‑free), 1/3 of the ricotta mixture dolloped and spread, 1/3–1/2 cup sauce, and a light sprinkling (about 1/3) of shredded mozzarella. Repeat two more times (noodles → ricotta → sauce → mozzarella). Finish with a final noodle or eggplant layer, remaining sauce spread evenly, remaining mozzarella, and the remaining 1/2 cup Parmesan. If you like a browned top, add an extra handful of cheese.
7. Cover and bake: Cover the dish tightly with foil (spray foil with cooking spray or tent to avoid sticking). Bake at 375°F (190°C) for 25 minutes. Remove foil and bake another 15–20 minutes until bubbly and golden on top. If the top browns too slowly, broil 1–2 minutes—watch closely.
8. Rest: Let the lasagna rest 10–15 minutes before slicing to help it set and make clean slices.
9. Serve: Garnish with chopped fresh basil or parsley and toasted pine nuts if desired. Serve with a green salad and crusty bread.
10. Store: Refrigerate leftovers up to 4 days; reheat single portions in the oven or microwave. Freeze portions up to 3 months—thaw overnight and reheat covered in a 350°F (175°C) oven until warmed through.

Tips and variations

• Make it gluten‑free by omitting noodles and using extra roasted eggplant layers or gluten‑free lasagna noodles.
• For a creamier, richer version, stir 1/2 cup shredded mozzarella into the ricotta mixture.
• Add lentils or cooked farro to the sauce for extra protein and texture.
• Use thinly sliced butternut squash or zucchini as alternative layers.

This Vegetarian Eggplant Lasagna is rich, satisfying, and vegetable‑forward — elegant enough for guests and comforting enough for weeknight dinner.