Vegetarian lasagna with whole-wheat noodles, sauteed spinach, part-skim ricotta, marinara, and mozzarella. About 375 calories and 21 grams of protein per square.
Preheat the oven to 375 F (190 C). Lightly oil a 9-by-13-inch baking dish.
If using regular lasagna noodles, bring a large pot of salted water to a boil, cook the noodles 2 minutes less than the package direction (they will finish in the oven), drain, and lay flat on an oiled sheet pan to prevent sticking. Skip this step for no-boil noodles.
Heat the olive oil in a large skillet over medium heat. Add the onion and cook 4 minutes until soft. Add the garlic and cook 30 seconds until fragrant.
Add the spinach in batches, tossing until each addition wilts. Cook 2 more minutes to drive off moisture, then transfer to a fine-mesh strainer and press hard with a spatula to remove excess water. Season with salt, pepper, and nutmeg. Set aside to cool for 5 minutes.
In a medium bowl, stir together the ricotta, egg, 1/4 cup of the Parmesan, parsley, basil, and the cooled spinach mixture. Taste and add a pinch more salt if needed.
Spread 3/4 cup of marinara across the bottom of the baking dish. Arrange 3 lasagna noodles on top in a single layer.
Spread half the ricotta-spinach mixture over the noodles. Top with 3/4 cup marinara and 2/3 cup mozzarella. Add a second layer of 3 noodles.
Spread the remaining ricotta-spinach mixture, then another 3/4 cup marinara and 2/3 cup mozzarella. Top with the final 3 noodles.
Pour the remaining marinara over the top layer, spreading to cover the noodles completely. Finish with the last 2/3 cup mozzarella and the remaining 1/4 cup Parmesan.
Cover tightly with foil (spray the underside first so cheese does not stick) and bake for 30 minutes. Remove the foil and bake another 15 to 20 minutes, until the top is bubbly and deeply golden.
Let the lasagna rest at least 15 minutes before slicing. This resting step is critical for clean squares. Cut into 8 pieces.