Using a mandoline slicer or sharp knife, cut the yellow squash into 1/8-inch thick rounds. Lay the slices out between layers of paper towels, sprinkle lightly with salt, and allow them to rest for 15 minutes — this draws out excess moisture and helps prevent a watery lasagna. While the squash drains, preheat your oven to 375°F.

In a large bowl, mix together the ricotta, parmesan, fresh basil, garlic powder, red pepper flakes, salt, black pepper, egg, and lemon zest. Stir until the mixture is smooth and evenly combined. In a separate microwave-safe bowl, microwave the fresh spinach with a splash of water for 3 minutes until fully wilted, then transfer it to a clean kitchen towel and squeeze out as much liquid as possible — thoroughly dry spinach is key to keeping the layers from becoming soggy.

Spread about 1/3 cup of marinara sauce in an even layer across the bottom of a 9x13 inch baking dish. Arrange one-third of the salted squash slices on top, then spread half of the ricotta mixture evenly over the squash, followed by half of the wilted spinach. Add another drizzle of marinara sauce and sprinkle with one-third of the shredded mozzarella.

Build a second layer starting with another third of the squash slices, followed by the remaining ricotta mixture, the remaining spinach, more marinara sauce, and another third of the mozzarella. Finish the lasagna with the final layer of squash slices, the remaining marinara sauce spread evenly on top, and the last of the mozzarella sprinkled over everything.

Cover the baking dish tightly with aluminum foil and bake at 375°F for 30 minutes. Remove the foil and continue baking uncovered for an additional 15 to 20 minutes, until the top is golden brown and the cheese is bubbling at the edges.

Take the lasagna out of the oven and let it rest, uncovered, for 15 minutes before slicing. This resting time allows the layers to firm up so each portion holds its shape when served. Sprinkle with dried parsley before serving.