01 - Preheat the oven to 425°F and line a large baking tray with parchment paper.
02 - Toss eggplant, zucchini, both bell peppers, and mushrooms with olive oil, oregano, thyme, salt, and pepper. Spread on the tray and roast for 25 minutes, stirring once halfway, until golden and tender. Set aside.
03 - Reduce oven temperature to 350°F.
04 - Melt butter in a saucepan over medium heat. Whisk in flour and cook for 2 minutes. Gradually add warm milk, whisking constantly until smooth. Simmer for 5 to 7 minutes, stirring until thickened. Season with nutmeg, salt, and white pepper.
05 - Spread a thin layer of béchamel in a 9x13 inch baking dish. Arrange 3 lasagna noodles over the sauce. Add half the roasted vegetables, a handful of spinach, dollops of ricotta, one-third each of mozzarella and parmesan, and a generous layer of béchamel.
06 - Repeat with 3 more noodles, remaining vegetables, spinach, ricotta, one-third of the cheeses, and béchamel.
07 - Top with last 3 noodles, remaining béchamel, and the rest of mozzarella and parmesan.
08 - Cover with foil and bake for 25 minutes.
09 - Remove foil and bake for an additional 15 to 20 minutes until the top is golden and bubbling.
10 - Let stand for 10 minutes before slicing and serving.