Preheat oven to 400 degrees. Thinly slice the eggplant and leave the skin on. Place the eggplant on paper towels to draw out any moisture and sprinkle lightly with salt. Let it sit for 10 minutes and then dab with paper towels to absorb any remaining moisture. Place the eggplant flat on a baking sheet.
In a large mixing bowl, place the chopped veggies, 2 Tbsp olive oil, minced garlic, and mix well. On a separate baking sheet, (for best flavor), spread out the chopped veggies and roast in the oven at 400 degrees for 20-25 minutes. Roast the eggplant on a separate shelf if they don't both fit on top shelf of the oven for 20-25 minutes at 400 degrees. (If you don't have time to roast the veggies in the oven with the eggplant, sauté the veggies in a cast iron skillet instead.)
While the eggplant and veggies are roasting, prepare the cheese mixture. Place the ricotta cheese, chopped spinach, Italian seasoning, garlic salt, and egg in a bowl and mix. Once the veggies and eggplant are finished roasting, start layering the lasagna.
For best layering, use a deep baking dish, typically a 9x13 pan is deep enough. There's no rule to layering, but I like to drizzle the marinara sauce on the bottom of my pan to start, then the eggplant , followed by cheese mixture, then roasted veggies, then sauce. Repeat with the eggplant, cheese, veggies, sauce ending with eggplant. Sprinkle mozzarella cheese on top and bake for 25 minutes at 350 degrees or until bubbly. The veggies are already cooked so bake until hot throughout.