Sauté your diced onions in the olive oil at low medium heat, until they have browned and caramelized. Next, add your Italian sausage and cook/stir at a low medium heat, bumping up the heat slightly if needed. Set aside when done placing it on a plate covered with paper towels to pull the grease out.
Make your homemade lasagna noodles using my recipe OR skip this step and use store bought lasagna noodles. It can be challenging if you are doing this gluten-free because not all stores carry gluten-free lasagna noodles. Just know they are crazy easy to make, but they do create an additional step. If you are using store bought noodles that need to be boiled, do that and set aside. If they are gluten-free a little olive oil will help to keep them from sticking. Some noodles do not require boiling so be sure to check that. If you are making my noodles and they are fresh, they definitely do not need to be boiled.
Place the Italian sausage mixture back on the stove at low medium heat and add 1 tsp garlic powder, 1 tsp Italian seasoning, 2 tsp tomato powder w/1 tsp water OR 2 tbsp tomato paste, tomato pureed (either purchased or using blended diced tomatoes). Let simmer for a couple minutes, then add the 3 cups of tomato sauce. Stir and simmer for a few minutes.
Put together your masterpiece. Preheat the oven to 350 degrees. Butter a 9x13 pan. I use a glass Pyrex. Layer by beginning with a thin layer of the meat/tomato sauce mixture. Next, place down the noodles however it fits for you. I typically do noodles going long ways but you do you!!! Next, add the sauce and then the three cheeses. I use mozzarella, parmesan and ricotta OR cottage cheese. I typically end up with about three layers of everything. Your goal should be to have some meat sauce and cheese for the final topping! If you don't, simply shred a bit more mozzarella and sprinkle on the top.
So to be clear, the layers go like this: first layer sauce, then noodles, sauce, cheeses, noodles, sauce, cheeses, noodles, sauce, cheeses. If you have more layers than that, and can fit it all in go for it!!!
Place your lasagna in the oven, covered with aluminum foil and bake for 30 minutes. Take off aluminum foil after thirty minutes and cook for an additional ten to fifteen minutes. It should be bubbling and even a little crispy around the edges. Take out of oven and let it rest for about ten minutes before serving. Delish.