01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or butter.
02 - Heat olive oil in a large skillet over medium heat. Add sliced leeks and minced garlic; sauté for 2-3 minutes until softened and fragrant.
03 - Add zucchini, yellow squash, and asparagus pieces to the skillet. Cook for 5-6 minutes until just tender. Stir in peas and chopped 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. Gradually whisk in warm milk, continuing to stir to prevent lumps. Add nutmeg, salt, and white pepper. Cook, stirring frequently, until sauce thickens nicely, about 5-7 minutes. Remove from heat.
05 - In a small mixing bowl, combine ricotta cheese with 1/2 cup Parmesan cheese. Mix thoroughly and season lightly with salt and pepper.
06 - Spread a thin layer of béchamel sauce on the bottom of the prepared baking dish. Arrange a layer of lasagna noodles over the sauce. Spread 1/3 of the ricotta mixture over the noodles, followed by 1/3 of the sautéed vegetables. Drizzle with béchamel sauce and sprinkle with mozzarella cheese.
07 - Repeat the layering process two more times: noodles, ricotta mixture, vegetables, béchamel, and mozzarella. For the final layer, place noodles, spread remaining béchamel sauce over the top, and sprinkle with remaining mozzarella and Parmesan cheeses.
08 - Cover the baking dish tightly with aluminum foil. Bake for 30 minutes to allow flavors to meld and noodles to cook through.
09 - Remove foil and bake for an additional 15-20 minutes until the cheese is golden brown and bubbly. The lasagna should register at least 165°F in the center.
10 - Let the lasagna rest for at least 10 minutes before slicing. This allows the layers to set and makes serving much easier.