Brown the three meats hard. In a large pot or Dutch oven over medium-high heat, cook the sweet sausage, hot sausage, and ground beef together, breaking them up as they go, until deeply browned — let them build real color, don't just cook them gray. That browning is where a huge chunk of the flavor lives. Spoon off the excess grease, leaving a tablespoon or two behind.

Build the sauce base. Add the diced onion to the meat and cook until soft and translucent, about 5 minutes, then stir in the garlic and cook 1 minute more until fragrant. Add the crushed tomatoes, tomato paste, tomato sauce, and water, and stir until everything is combined.

Season and simmer low and slow. Stir in the sugar, basil, Italian seasoning, fennel seeds, red pepper flakes, salt, and pepper. Bring it to a gentle bubble, then drop the heat to low and simmer uncovered for at least 45 minutes (up to 1½ hours if you have the time), stirring now and then, until the sauce is thick and glossy. Stir in the fresh parsley at the end. This slow reduction is what keeps the finished lasagna from going watery.

Cook the noodles. Boil the lasagna noodles in well-salted water to just shy of al dente, drain, and lay them out flat in a single layer so they don't stick together. (Skip this step if you're using no-boil noodles.)

Mix the ricotta layer. In a bowl, stir together the ricotta, egg, Parmesan, parsley, and salt until smooth. The egg is the key — it sets the ricotta as it bakes so the layer turns into creamy pockets instead of running out the sides.

Layer it up. Spread a thin layer of meat sauce over the bottom of a 9x13-inch baking dish. Add a layer of noodles, then spread half the ricotta mixture over them, scatter on a third of the mozzarella, ladle over a third of the remaining sauce, and sprinkle with Parmesan. Repeat: noodles, the rest of the ricotta, another third of the mozzarella, more sauce, Parmesan. Finish with a final layer of noodles, the last of the sauce, the remaining mozzarella, and a good shower of Parmesan on top.

Bake covered, then uncovered. Tent a sheet of foil over the dish (tent it so it doesn't stick to the cheese) and bake at 375°F for 25 minutes. Uncover and bake another 25 minutes, until the sauce is bubbling around the edges and the cheese on top is golden and blistered in spots.

Rest it — don't skip this. Let the lasagna rest 15 to 20 minutes before cutting. This is the most-skipped step and the most important one at the finish: cut it straight out of the oven and it slides apart into a saucy puddle; give it that rest and it sets into clean, sturdy squares that hold their shape on the plate.