01 - Preheat oven to 425°F. Line a large baking tray with parchment paper.
02 - Toss eggplant, zucchini, bell peppers, and mushrooms with olive oil, oregano, thyme, salt, and pepper. Spread evenly on the tray and roast for 25 minutes, stirring once halfway, until tender and golden. 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 whisk in warm milk until smooth. Simmer for 5-7 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. Top with half the roasted vegetables, a handful of spinach, dollops of ricotta, one-third of the mozzarella and parmesan, and a layer of béchamel.
06 - Repeat layering with 3 noodles, remaining roasted vegetables, spinach, ricotta, another third of the cheeses, and béchamel.
07 - Top with final 3 noodles, remaining béchamel, and the remaining mozzarella and parmesan cheeses.
08 - Cover with foil and bake for 25 minutes.
09 - Remove foil and bake for an additional 15-20 minutes until the top is golden and bubbling.
10 - Let rest for 10 minutes before slicing and serving.