Step 1: Brown the Beef and Onion - Start with a deep skillet over medium heat and add the ground beef and diced onion. Cook them together for 8 to 10 minutes, breaking the beef into small crumbles as it cooks. I like doing the onion right in with the meat because it softens in all those juices and gives the sauce a better base. Once the beef is no longer pink and the onion looks tender, stir in the minced garlic and cook it for about 1 minute. Garlic can burn quickly, so do not walk away here. Just let it wake up and smell good.
Step 2: Add the Chicken Sausage - Stir the cooked and crumbled Italian chicken sausage into the skillet. Since the sausage is already cooked, you are really just warming it through and letting it join the party. The combination of beef and chicken sausage gives this homemade lasagna a nice balance. You get the hearty beef flavor, plus the seasoned sausage flavor, without the sauce feeling too greasy or heavy.
Step 3: Simmer the Tomato Meat Sauce - Add the crushed tomatoes, tomato paste, tomato juice or water, beef broth, brown sugar, 1/4 cup parsley, dried basil, oregano, fennel seed, 1/2 teaspoon salt, and pepper. Stir it all together until the tomato paste loosens into the sauce. Bring it to a boil, then reduce the heat and let it simmer uncovered for 30 minutes, stirring now and then. This is where the sauce turns from a bunch of ingredients into something that tastes like dinner. It thickens, the herbs soften, and the fennel gives it that old school lasagna smell that brings people into the kitchen.
Step 4: Mix the Cheese Filling - In a bowl, mix the lightly beaten egg, ricotta cheese, cottage cheese, shredded mozzarella, and Parmesan cheese. Stir it until everything is evenly combined. The egg helps the filling hold together so your slices do not completely collapse when you serve them. Now, I am not going to pretend lasagna always cuts like a restaurant photo. Sometimes the first piece is the cook’s piece, and that is just how life works. But this cheese mixture helps it scoop nicely and taste rich in every bite.
Step 5: Chop the Spinach - Finely chop the spinach if you are using it. I like cutting it small so it tucks into the layers instead of pulling out in big strands. Spinach is optional, but it gives the lasagna a little color and makes me feel like I did something responsible while also adding five cheeses. Fresh spinach works well here, and if it seems damp after chopping, pat it lightly with a paper towel so it does not water down the layers.
Step 6: Build the First Layers - Preheat the oven to 375°F and grease a 13x9 inch baking dish. Spread a thin layer of meat sauce over the bottom of the dish. This keeps the noodles from sticking and gives them moisture right away. Add half of the uncooked lasagna noodles over the sauce. Break a noodle if you need to make things fit. This is family cooking, not tile work. Spoon another thin layer of meat sauce over the noodles so they are covered.
Step 7: Add Cheese and Spinach - Drop clumps of the cheese mixture over the sauce, then gently spread it a bit with the back of a spoon. It does not have to be perfect. As it bakes, the cheese will melt and settle into the spaces. Add half of the chopped spinach over the cheese if you are using it. This layer is where you start seeing the lasagna become something special, with sauce, pasta, creamy cheese, and green all tucked together.
Step 8: Finish the Layers - Add the remaining noodles, more meat sauce, the rest of the cheese mixture, and the remaining spinach. Make sure the noodles have enough sauce around them because that moisture is what cooks them in the oven. Finish with the sliced fresh mozzarella across the top. The dish will be full, and that is exactly right. Put the baking dish on a rimmed sheet pan if you are worried about bubbling over. That little dad move saves you from cleaning the oven later.
Step 9: Bake Until Bubbly - Bake the lasagna uncovered for 50 to 60 minutes, until the sauce is bubbling around the edges, the noodles are tender, and the cheese on top looks melted and lightly golden in spots. If the top starts browning faster than you like, loosely cover it near the end. Once it comes out of the oven, let it stand for 15 minutes before slicing. I know that is the hardest part, but it really does help the layers settle. Finish with the remaining parsley and extra Parmesan if you like.