For the meat sauce

1. In a large saucepan over medium-high heat, brown the sausage and beef until cooked through. If excess fat has rendered you can drain this, then reserve meat to the side in a separate dish.
2. Return the same pan to the stove and add garlic oil (or olive oil + minced garlic) and the onion. If using a traditional white onion, you'll want to sautee until translucent. If using green onions, you can sautee for about a minute before moving on to the next step.
3. Add the tomato paste and stir to combine. Allow to cook, stirring often until the tomato paste has darkened in color, then add the tomato sauce and stir, gently scraping to make sure that any residual brown bits from the meat and tomato paste get stirred in.
4. Return the meat to the pan with the tomato sauce, season with salt, pepper, basil, and oregano, stirring to combine. Reduce the heat to medium-low and allow the sauce mixture to come to a simmer. Taste for salt and adjust if needed. Continue to simmer for about 30 minutes until the sauce reduces slightly, stirring occasionally.

For the spinach ricotta filling

1. In the meantime, in a medium saucepan over medium heat, heat the olive oil. Add about 1/4th of the spinach to the pan and sautee until it begins to wilt, then continue with an additional 1/4th. You'll want to work in batches to add the spinach as there is so much of it, but it wilts quickly! If you find that you need a bit more moisture to keep the spinach cooking without drying out you can add about a tbsp of water to the pan while cooking, but don't add too much as spinach contains quite a bit of water on its own and you do not want soggy spinach as it will make the ricotta filling soggy. Once the spinach has wilted and exponentially decreased in size, remove from heat and allow to cool.
2. Once the spinach is cool, add to a food processor along with the ricotta cheese, parmesan cheese, eggs, salt, and red pepper flakes. Blend until the spinach and eggs have broken down and combined with the ricotta. The mixture should be light green in color. Reserve to the side until ready to layer.

Assembly

1. Spray a 9 x 13-inch casserole dish (or (2) 8 x 8-inch casserole dishes *see notes) with cooking spray or grease with a light layer of olive oil, including the inner-sides of the pan. Ladle about 1/2 cup meat sauce in the bottom of the pan, spreading evenly to coat.
2. Layer 1: Layer lasagna sheets in an even layer, taking care not to overlap the pasta. You may have to break some of the lasagna pieces in order to fill the pan. If using the brand I linked to, you'll need to break a few of them into fourths in order to fill the pan. I find that the easiest way to do this is to use a sharp knife to carefully make a kind of perforated cut into the dried pasta where you want it to break. This will help the pieces to break more evenly by hand. Each of my layers had 3 sheets of lasagna placed horizontally in the vertical pan and 2 additional 1/4th pieces at the bottom edge. Layer half of the remaining meat sauce on top of the noodles, ensuring that the sauce is spread in a reasonably even layer. Sprinkle the top of the sauce with 1/4th of the mozzarella cheese and 1/4th of the parmesan cheese.
3. Layer 2: Layer the second layer of lasagna sheets in an even layer, just as you did in the previous layer. Top with 1/2 of the spinach ricotta filling, then sprinkle with another quarter portion of mozzarella cheese and parmesan cheese.
4. Layers 3 and 4: Repeat layers 1 and 2 until you have 4 alternating layers: 2 with meat sauce and 2 with spinach ricotta filling. Top the lasagna with any remaining mozzarella cheese and parmesan cheese.
5. Allow the lasagna to cool if still warm, then generously spray a layer of tin foil with cooking spray or grease lightly with olive oil and top the pan. Store in the fridge overnight if making the following day or store in the freezer if prepping ahead of time.
6. When ready to bake, preheat the oven to 400 degrees F / 205 C. Set the baking dish on a large rimmed baker's half sheet to catch any over-bubbling sauce. If the lasagna is still in its thawed refrigerated state, bake for 45-minutes, then carefully remove the foil and bake for an additional 15-minutes, until the cheese on top is golden brown and bubbling. If baking from frozen, the lasagna will need to bake for at least 75-minutes with the foil and then for 15-minutes after the foil has been removed. Allow the lasagna to cool for 15-minutes before slicing and serving.