In your cast-iron skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes. Then add the minced garlic and cook for an additional minute until fragrant.
Stir in the sliced mushrooms and diced zucchini. Sauté for about 5 minutes, or until the mushrooms release their moisture and the zucchini is tender. Add the baby spinach leaves and cook until wilted. Season with salt and pepper to taste.
Pour in the Italian tomato sauce, stirring everything until well combined. Let the mixture simmer for a few minutes to meld flavors.
Break the lasagna noodles into 3-4 inch pieces and add them to the skillet. Stir to ensure noodles are submerged in the sauce.
Dollop the ricotta cheese over the top of the noodle mixture. Then sprinkle shredded mozzarella and Parmesan cheese evenly across the top.
Preheat your oven to 375°F (190°C). Transfer the skillet to the oven and bake for about 25-30 minutes, or until the cheese is bubbly and golden brown.
Once out of the oven, let the lasagna rest for about 5-10 minutes to help layers set and make serving easier.
Slice into your skillet lasagna, serve warm, and enjoy this cheesy, veggie-packed dish. Pair with a side salad for a complete meal!