Place 2 large eggplants on cutting board. Slice off both ends of each eggplant, then stand one eggplant on one end. Starting at top end of eggplant and cutting straight down, carefully slice eggplant into long, even pieces, each approximately ⅛-inch to ¼-inch thick. Repeat with other eggplant.
Layer paper towels on flat work surface, then place wire cooling rack(s) over paper towels. Alternately, fit wire cooling rack(s) on top of large sheet pan(s).
Lay each slice of eggplant flat on wire rack(s). Liberally sprinkle salt over each slice, then flip slices over and generously salt other side of each slice.
Let eggplant slices rest 10 to 15 minutes, allowing slices to sweat out as much moisture as possible. After 10 to 15 minutes, quickly rinse excess salt off of eggplant slices (be careful not to soak slices) then pat each slice of eggplant very dry with paper towels. Set aside.
To Prepare the Marinara
While eggplants sweat, add 1 28-ounce can crushed tomatoes, 1 teaspoon garlic powder, 1 teaspoon Italian seasoning, ½ teaspoon crushed red pepper flakes, and 1 pinch salt to large mixing bowl. Stir all ingredients together until well incorporated. Set aside.
To Prepare the Ricotta Filling
While eggplants sweat, crack 1 large egg directly into medium mixing bowl. Whisk egg vigorously until egg is well beaten and pale in color, with few or no streaks remaining.
Add 1 15-ounce container ricotta, 1 ½ cups shredded fresh mozzarella, ½ cup grated fresh parmesan, ½ teaspoon fresh lemon zest, 2 cloves garlic, ½ teaspoon salt, 1 tablespoon chopped fresh basil leaves, and 10 ounces frozen spinach to mixing bowl with beaten egg. Gently stir ingredients together until fully incorporated. Set aside.
To Assemble the Lasagna
Preheat oven to 375° Fahrenheit. Position oven rack in center of oven. Lightly spray 9x13 baking dish with cooking spray.
Add 1 ½ cups of prepared marinara to baking dish. Spread marinara out into flat, even layer, covering bottom of baking dish completely.
Layer half of rinsed and dried eggplant slices on top of marinara in one even layer, slightly overlapping slices as needed to fit in baking dish.
Top eggplant slices with half of ricotta filling mixture. Spread ricotta filling out into one even layer, covering eggplant layer completely.
Spread 1 ½ cups prepared marinara evenly over entire surface of ricotta layer, then layer remaining eggplant slices over top of marinara.
Top eggplant with remaining ricotta mixture, spreading filling out to evenly cover eggplant slices.
Top ricotta layer with remaining marinara, spreading marinara out evenly to cover ricotta filling. Sprinkle ½ cup shredded fresh mozzarella evenly across top of marinara, then sprinkle ½ cup grated fresh parmesan evenly across top of mozzarella.
To Bake the Lasagna
Place baking dish in preheated oven. Bake lasagna, uncovered, 45 minutes, until mozzarella and parmesan are completely melted.
Once cheeses have melted, turn oven to broiler and set heat to High if applicable. Broil lasagna 5 minutes or until cheese topping is just browned and bubbly.
When satisfied with doneness of cheese layer, carefully remove baking dish from oven and set aside. Let lasagna rest in baking dish 15 to 20 minutes or until lasagna has set.
After lasagna has rested, sprinkle 2-3 tablespoons finely chopped fresh basil leaves across top of lasagna. Cut lasagna into 12 evenly-sized squares. Serve lasagna warm with garlic bread or small side salad if desired.
Recipe Notes
If you notice excess moisture in the bottom of the baking dish after you remove your first slice, don't panic. Just ball up a paper towel or two and place them in the spot where the lasagna square was. The paper towels will soak up that excess liquid in no time. You can tilt the pan a little to encourage some of the liquid to run toward the paper towels, but it's not necessary.
Eggplant: The number of slices you get out of each eggplant will depend on the size of the eggplant and the thickness of the slices you cut. I was able to get 6 quarter-inch-thick slices from each eggplant.
Cheeses: I recommend buying fresh mozzarella and fresh parmesan, then shredding and grating them yourself. Prepackaged, pre-shredded and pre-grated cheeses contain additives to prevent clumping which can also keep the cheese from melting like they should.
Make-Ahead Option: Prepare each component (the eggplant slices, the marinara, and the ricotta filling) no more than 24 hours in advance. Refrigerate each component separately in an airtight container. Assemble the lasagna no more than 2 hours ahead of baking. If you're not baking the assembled lasagna right away, cover the baking dish with plastic wrap and refrigerate it until you're ready to bake it, then let the lasagna sit at room temperature while the oven preheats.