In a large saucepan or Dutch oven over medium-high heat, add beef (or sausage) and onion, and cook until mostly browned, about 5 minutes. Drain if desired. Stir in garlic until fragrant, about 30 seconds.: You will hear the meat begin to sizzle and smell savory, meaty aromas as the edges brown and the onion softens, signaling the development of flavor. The onions should turn translucent and slightly golden, offering a gentle sweet perfume that balances the savory meat. The garlic will bloom quickly, releasing a warm, aromatic note in the pan, but pay attention because garlic can turn bitter if it burns. If the mixture seems oily, draining excess fat will prevent a greasy sauce later. A common mistake here is crowding the pan which causes steaming rather than browning, reducing the depth of flavor.
Stir in crushed tomatoes, tomato sauce, tomato paste, sugar, basil, Italian seasoning, and fennel seeds. Simmer uncovered for 15 minutes, stirring occasionally. Season to taste with salt and pepper (I like 1 teaspoon salt and 1/2 teaspoon pepper). You should have about 2 quarts of sauce.: As the sauce comes together, you will notice the aroma change from raw tomato to a deeper, more concentrated tomato scent; the surface will show gentle bubbles and steam. The paste thickens the body while the crushed tomatoes give texture, and sugar tames bright acidity for a rounder profile. The fennel seeds will perfume the sauce with a subtle anise whisper that complements the sausage. Stirring occasionally prevents the bottom from catching, and simmering uncovered concentrates flavors. Watch for the sauce sticking to the pan rim as a sign it needs a lower flame. Overreducing will dry the sauce and leave insufficient moisture for softening the noodles later.
Meanwhile, in a large bowl, combine ricotta cheese with egg, parsley, and salt. Chill until the sauce is finished.: The ricotta mixture should feel cool and creamy, with the egg folded evenly to create a binding custard that will set during baking. The dried parsley disperses flecks of herb throughout, and the salt brightens the dairy. Chilling it keeps the mixture firm so it layers neatly and prevents it from loosening when assembled. One error people make is overwhipping the ricotta which can introduce air and alter texture, so fold gently for a smooth, slightly dense filling.
In the bottom of a 9-inch x 13-inch baking dish, spread 2 cups meat sauce. Arrange a single layer of uncooked noodles over the meat sauce (I sometimes do 3 full-size noodles lengthwise and then a partial noodle crosswise). Spread with a heaping 2/3 cup of the ricotta mixture. Top with about 1 1/2 cups (5 ounces) of mozzarella. Sprinkle with 1/3 cup Parmesan cheese.: When you spread the initial sauce, it should look glossy and slightly loose; this moisture is crucial so the noodles can hydrate. Lay the noodles so they overlap minimally but cover the sauce, and the ricotta should sit like a creamy island that contrasts the red. The mozzarella should be scattered evenly so it melts uniformly, and the Parmesan creates a salty lift. A telltale sign of correct layering is the pan feeling balanced, not overly damp or dry. Avoid piling too much ricotta in one spot, which can lead to pockets that do not heat through evenly.
Spoon 2 cups meat sauce over the cheese. Make another layer of noodles, ricotta mixture (heaping 2/3 cup), mozzarella (1 1/2 cups) and Parmesan (1/3 cup). Repeat layers 1 more time, for a total of 3 layers.: Each added layer should sit atop the previous one with a little give, and you should see distinct strata of red and white through the side of the pan. The smell will become richer as the assembled layers trap the sauce aromas, and the pan should feel substantial but not overloaded. Layering evenly ensures every slice has a balance of sauce, cheese, and noodle. A frequent mistake is uneven layering which causes some pieces to fall apart while others are dense, so take a moment to level each layer as you build.
Spray a large piece of foil with nonstick spray and cover baking dish. Refrigerate at least 5 hours or overnight.: Chilling allows the sauce and cheese to meld, and you may notice the surface firms slightly as the fats cool, which helps slicing later. Refrigeration also gives the noodles time to hydrate in the sauce, so they finish perfectly when baked. When covering, ensure the foil is secure to prevent fridge drying or odor transfer. One pitfall is refrigerating shorter than recommended which can leave the noodles underhydrated and the texture less cohesive after baking.
Preheat oven to 375 degrees. Bake, covered with foil, for 25 minutes. Remove foil and bake until hot and bubbly, about 25 minutes longer. Cool 5 minutes, then garnish with parsley before serving if desired.: When baking, the kitchen will fill with a warm tomato and toasted cheese aroma, and you should see steam escaping around the edges; bubbling indicates the sauce is fully heated. Removing the foil allows the top to brown and the cheeses to develop golden patches, while keeping it covered initially prevents the top from drying before the center heats. Use a thermometer to ensure the center reaches a safe temperature if needed. A common mistake is skipping the rest period, which results in runny slices; letting it cool for a few minutes helps the layers set for cleaner pieces.