Preheat the oven to 375°F (190°C). Lightly coat a 9"x13" baking pan with cooking spray.

Bring a large pot of salted water to a boil and cook the lasagna noodles according to the package directions until al dente. Drain and lay noodles flat on a sheet of parchment or a lightly oiled tray to prevent sticking.

While the noodles cook, melt the 2 tablespoons unsalted butter in a large skillet over medium-high heat.

Add the 24 ounces fresh spinach leaves to the skillet in batches, tossing each batch until just wilted. Remove the wilted spinach to a large bowl and repeat until all spinach is cooked. Do not add additional salt or pepper while cooking.

Let the cooked spinach cool slightly, then press or squeeze out as much excess water as possible (use your hands, a clean kitchen towel, or cheesecloth). Coarsely chop the drained spinach and return it to the bowl.

In a separate bowl, lightly beat the 2 room-temperature eggs. Add the beaten eggs to the chopped spinach.

Add to the spinach-and-egg mixture: 1 cup chopped fresh basil leaves, 4 cloves minced garlic, 14 ounces ricotta cheese, 1 1/2 cups grated Parmesan cheese, 1 cup shredded Mozzarella cheese, 1 teaspoon Italian seasoning, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Stir until thoroughly combined to form the cheese mixture.

Spread a thin layer (about 1/2 to 3/4 cup) of the 24-ounce jar of marinara sauce on the bottom of the prepared 9"x13" pan.

Assemble the lasagna: place a single layer of cooked lasagna noodles over the sauce, spread 1/3 of the cheese mixture evenly over the noodles, then spoon 1/3 of the remaining marinara sauce over the cheese mixture. Repeat this sequence two more times (noodles → 1/3 cheese mixture → 1/3 sauce), ending with the final portion of marinara sauce on top.

Cover the pan tightly with foil and bake in the preheated oven for 40–45 minutes.

Remove the foil, sprinkle the top with the remaining 2 cups shredded mozzarella cheese, and return the lasagna to the oven. Bake uncovered for an additional 15 minutes, or until the cheese is melted, bubbly, and lightly browned.

Remove the lasagna from the oven and sprinkle with 2 tablespoons chopped parsley and the reserved 2 tablespoons chopped basil.

Let the lasagna stand for 10–15 minutes before cutting into slices and serving.