01 - Preheat oven to 375°F.
02 - In a large skillet, heat olive oil over medium heat. Add zucchini, squash, asparagus, leek, and peas. Sauté for 5-7 minutes until just tender. Stir in spinach to wilt. Season with salt and pepper. Remove from heat and set aside.
03 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute. Gradually add milk, whisking constantly to prevent lumps. Bring to a gentle simmer and cook until thickened, about 5 minutes. Stir in nutmeg, salt, and white pepper. Remove from heat.
04 - In a small bowl, combine ricotta, egg, and 1/2 cup Parmesan. Mix well.
05 - Spread a thin layer of béchamel on the bottom of a 9x13-inch baking dish. Layer 3 noodles over sauce. Top with 1/3 of the ricotta mixture, 1/3 of the sautéed vegetables, 1/3 of the mozzarella, and 1/4 of the béchamel. Repeat layers twice more. Finish with remaining noodles, béchamel, mozzarella, and Parmesan.
06 - Cover with foil and bake for 30 minutes.
07 - Remove foil and bake an additional 15-20 minutes, or until golden and bubbling. Let rest for 10 minutes before slicing and serving.