Veggies: Heat 1 Tbsp olive oil in a large saute pan over medium heat. Add 1 eggplant, 1 zucchini, and 1 red bell pepper (all diced), cooking until veggies are soft, about 10 minutes. Add 1 handful spinach and 2 cloves garlic (minced) and cook, stirring constantly, until wilted down, about 1 minute.
Bechamel: Heat 2 cups milk in the microwave for about 2 minutes until warm. Melt 4 Tbsp unsalted butter in a heavy-bottomed pan over medium heat. Whisk in the ¼ cup all-purpose flour, cooking and stirring until the mixture is slightly golden brown and no longer smells like raw flour, about 2 minutes. Slowly whisk in all of the milk, stirring well to combine evenly. Bring the mixture to a simmer and cook until thickened, about 5 minutes. Remove from heat and stir in ½ cups grated parmesan cheese and ¼ tsp each salt and pepper
Assemble: Preheat oven to 350°F (175°C). Spread ½ cup of marinara onto the bottom of a 9x13-inch casserole dish. Layer in ⅓ of the lasagna noodles.Then layer - ⅓ of the bechamel, half of the veggies, ⅓ of the cheese, and 1 cup of tomato sauce. Top with ⅓ of the lasagna noodles.Repeat for the second layer - ⅓ of the bechamel, the second half of the veggies, ⅓ of the cheese, and 1 cup of tomato sauce. Top with ⅓ of the lasagna noodles.For the top - Spoon on the remaining bechamel and tomato sauce, then sprinkle on the remaining cheese.
Bake: Covered tightly with aluminum foil and bake for 45 minutes until bubbling at the edges. Remove foil, increase temperature to 400°F (204°C), and continue cooking until the top is golden brown.
Rest: Allow lasagna to sit uncovered on the counter for 15 to 45 minutes before serving to help layers firm up and avoid a soupy mess.
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Notes
This lasagna can be frozen before or after cooking! Here's a reader note on what worked well for her:"As a tip to anyone who makes this, make two and freeze the second. We prepped everything in the pan, then covered it with plastic wrap and foil before popping the whole thing in a 2-gallon freezer bag. It froze and reheated super well. Just be sure to add additional time since, you know, it’s frozen."