Slice the yellow squash into 1/8-inch thick rounds—a mandoline slicer makes this quick and uniform. Arrange the slices between paper towels and salt them lightly, then let sit for 15 minutes. This draws out excess moisture so your lasagna won't be watery. While the squash sits, preheat your oven to 375°F.

In a large bowl, combine the ricotta, parmesan, fresh basil, garlic powder, red pepper flakes, salt, black pepper, egg, and lemon zest. Stir until smooth and well combined. In a separate bowl or microwave-safe container, microwave the fresh spinach with a splash of water for 3 minutes until wilted, then squeeze it very dry in a clean kitchen towel to remove excess liquid—this prevents soggy lasagna. I like to add the lemon zest to the ricotta because it brightens the savory flavors without making the dish taste acidic.

Spread a thin layer of marinara sauce (about 1/3 cup) on the bottom of a 9x13 inch baking dish. Layer one-third of the salted squash slices, then spread half of the ricotta mixture from Step 2, followed by half of the wilted spinach from Step 2. Drizzle with more sauce and sprinkle with one-third of the mozzarella. Repeat with another layer of squash, remaining ricotta, remaining spinach, sauce, and another third of the mozzarella. Finish with the final squash layer, remaining sauce, and the last of the mozzarella on top. I find that ending with cheese helps it brown beautifully and creates a golden crust.

Cover the lasagna with foil and bake for 30 minutes at 375°F. Remove the foil and bake uncovered for an additional 15-20 minutes until the top is golden brown and cheese is bubbling at the edges. Remove from the oven and let rest for 15 minutes before serving—this allows the layers to set so each slice holds together cleanly.