01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or cooking spray.
02 - In a medium saucepan over medium heat, melt butter. Whisk in flour and cook for 1-2 minutes until pale golden, stirring constantly. Gradually whisk in milk, ensuring no lumps form. Add nutmeg, salt, and pepper. Simmer, stirring continuously, until sauce thickens (5-7 minutes). Remove from heat and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add asparagus, zucchini, and scallions; sauté for 4 minutes. Add garlic, spinach, and peas; cook another 2-3 minutes until vegetables are just tender. Season lightly with salt and pepper. Remove from heat.
04 - In a medium bowl, combine ricotta cheese, egg, 1/2 cup Parmesan, basil, parsley, salt, and pepper. Mix until smooth and well incorporated.
05 - Spread a thin layer of béchamel sauce on the bottom of the prepared baking dish. Arrange 3-4 lasagna noodles on top, slightly overlapping if necessary.
06 - Spread 1/3 of the ricotta mixture over noodles, followed by 1/3 of the sautéed vegetables, 1/3 of the remaining béchamel, and 1/3 of the mozzarella. Repeat layers two more times. Finish with remaining béchamel, mozzarella, and sprinkle with remaining 1/2 cup Parmesan.
07 - Bake for 40-45 minutes until top is golden brown and sauce is bubbling around edges. If desired, broil for 2-3 minutes for a crisped top layer, watching closely to prevent burning.
08 - Let lasagna rest for 10-15 minutes before slicing. This allows layers to set for cleaner portions.