First things first. Preheat your oven to 375°F (190°C). We want it to be nice and hot, ready for our fantastic creation.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the finely diced onion and sauté until translucent. Toss in the ground beef and Italian sausage, breaking it apart with a spoon. Cook until browned, seasoning it with salt, pepper, and Italian seasoning. You want all those flavors to meld together beautifully.

In a large mixing bowl, combine the cooked meat mixture, marinara sauce, and ricotta cheese. Go ahead and fold in the thawed spinach, ensuring it’s evenly distributed throughout. Add the crushed red pepper flakes if you want that touch of warmth.

Now, grab a large sheet pan and start layering. Spread a thin layer of the meat mixture at the bottom. Now, sprinkle a layer of broken lasagna noodles. Don’t worry about making it perfect—rustic is beautiful in this dish. Continue layering until you run out of meat mixture and noodles.

Once you’ve completed your layers, it's time to top it off. Generously sprinkle the shredded mozzarella and grated parmesan cheese over the top layer. You can't go wrong with cheese; the more, the merrier!

Cover the pan with aluminum foil to prevent the cheese from browning too soon. Bake in the preheated oven for about 30 minutes. After that, remove the foil and bake for another 15-20 minutes until the cheese is bubbly and golden brown.

After removing it from the oven, let your sheet pan lasagna rest for about 10–15 minutes. This step is crucial. It allows the ingredients to set and makes slicing easier.