01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with butter or cooking spray.
02 - Heat olive oil in a large skillet over medium heat. Add leeks and garlic, sauté for 2-3 minutes until softened and fragrant.
03 - Add zucchini, yellow squash, and asparagus to the skillet. Cook for 5-6 minutes until just tender. Stir in peas and spinach, cook for 1-2 minutes until spinach wilts. Season generously with salt and pepper. Remove from heat and set aside.
04 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute, stirring constantly to form a roux. Gradually whisk in warm milk, stirring continuously to prevent lumps. Add nutmeg, salt, and white pepper. Cook for 5-7 minutes, stirring constantly, until sauce thickens enough to coat the back of a spoon. Remove from heat.
05 - In a small bowl, combine ricotta cheese with 1/2 cup Parmesan cheese. Season lightly with salt and pepper, mixing until smooth and well incorporated.
06 - Spread a thin layer of béchamel sauce on the bottom of the prepared baking dish. Arrange a layer of noodles over the sauce. Spread 1/3 of the ricotta mixture over noodles, top with 1/3 of the sautéed vegetables, drizzle with béchamel, and sprinkle with mozzarella. Repeat layers two more times, finishing with a final layer of noodles, béchamel, remaining mozzarella, and remaining Parmesan.
07 - Cover baking dish tightly with aluminum foil. Bake for 30 minutes to allow flavors to meld and pasta to cook through.
08 - Remove foil and bake for an additional 15-20 minutes until cheese is golden brown and sauce is bubbling around the edges. Remove from oven and let rest for 10 minutes before slicing to allow layers to set.