How to Make It
Preheat oven to 400°F. Line 2 baking sheets with parchment paper. Working with 1 eggplant at a time, trim ends off eggplant; trim outside 4 edges off eggplant, creating a rectangular shape with flat sides. Chop trimmed pieces to equal 1 1/2 cups, and set aside. (Discard remaining trimmed pieces, or reserve for another use.) Using a mandoline, thinly slice eggplant “rectangles” lengthwise into 1/4-inch-thick slabs. (You should have about 20 eggplant slabs total.)
Heat oil in a large nonstick skillet over medium-high. Add onion and reserved chopped eggplant; cook, stirring often, until vegetables are tender, 8 to 10 minutes. Add 1 1/4 cups of the marinara sauce; stir to combine. Remove from heat; set aside.
Arrange eggplant slabs in a single layer on prepared baking sheets. Cover each baking sheet with aluminum foil. Bake in preheated oven until eggplant are pliable and tender but not fully cooked, about 8 minutes. Remove from oven; let cool 5 to 10 minutes.
Meanwhile, stir together egg, spinach, ricotta, garlic, salt, and 1 cup of the mozzarella in a bowl until combined.
Spoon remaining 1/4 cup marinara sauce over bottom of a 13- x 9-inch baking dish. Place 1 eggplant slab on a flat surface; spoon about 1 1/2 tablespoons spinach-ricotta mixture onto the widest end of the slab, then roll up toward the opposite end. Place eggplant roll, seam side down, on top of marinara in baking dish. Repeat process with remaining eggplant slabs and spinach-ricotta mixture, creating 3 rows of 6 to 7 rollatini in baking dish. Spoon onion-eggplant-marinara mixture over rollatini; sprinkle with remaining 1/2 cup mozzarella.
Cover baking dish with aluminum foil. Bake at 400°F until cheese and sauce are bubbly and rollatini are very tender, about 45 minutes. Remove and discard foil; continue baking 10 minutes. Remove from oven; let stand 5 minutes. To serve, place 4 rollatini and 1/3 cup sauce on each of 5 plates; sprinkle evenly with basil.