Preheat the oven to 400℉ and have a large oven-safe skillet ready (I use my 10-inch cast iron skillet for this recipe).
Warm the 2 tablespoons olive oil in the skiillet over low heat, then add the 4 cloves garlic and cook for about 1 minute, until fragrant. Stir in the 3 cups spinach until wilted, then add the 3 cups tomato sauce, season with salt and pepper and ½ teaspoon dried oregano, and simmer for 5 minutes.
Remove from the heat and take out about ⅔ of the sauce and set aside. Break the 8 pieces gluten free lasagna noodles into pieces, and layer one-half of the noodles over the sauce. Top with half of the 1 ½ cups ricotta cheese and ½ cup of the 1 cup shredded mozzarella. Pour half of the remaining sauce over, then repeat the process (noodle pieces, ricotta, and mozzarella). Finish with the remaining sauce, and sprinkle with ½ cup shredded Parmesan.
Cover the pan tightly with foil and place it in the oven. Bake for about 30 minutes, until the noodles are tender and the cheese is melted. Let the lasagna stand for at least 5 minutes out of the oven before serving.
Notes
Do I need to cook the noodles first? No! The beauty of this recipe is that the tomato sauce cooks the noodles while the lasagna bakes in the oven. No need to use no-boil noodles either.
Can I use other pasta shapes? Since this is more of a casserole, you could use other pasta shapes in place of the pasta, using the same method. I would use about 1 cup of noodles like penne, spirals, or ziti, and the cooking time may change a bit.
Use cottage cheese. For a lighter yet still high-protein version of this dish, substitute cottage cheese for the ricotta.
Uncover at the end. When the noodles are tender, you can uncover the pan and let it cook a few extra minutes uncovered. I like to do this so that the Parmesan on top gets nicely melted and even a bit browned (yum).
Let it sit. Like all lasagnas, skillet lasagna also benefits from resting when it comes out of the oven. At least 5 minutes is good, and 10 minutes is even better.