In a pot/Dutch oven, sauté the onion in the olive oil for 4 minutes, then stir in the ground beef and sausage meat, breaking it up with your spoon as you go along, and cook until meat is browned (about 8-10 min.). Spoon out most of the fat, but leave a few tablespoons in for flavor. Add in the remaining meat sauce ingredients. When it starts to bubble, reduce the heat (medium-low to low), cover the pot with the lid slightly open, and simmer for 30 minutes, stirring occasionally. Sauce should be quite thick.
While the sauce is cooking, gather the remaining ingredients. Add the egg to a fairly large prep bowl and beat it with a fork. Add in the rest of the cheese layer ingredients and stir until combined.
Preheat your oven to 375F and move the rack to the top third of the oven.
When the meat sauce is about 15-20 minutes from being done, boil a large, salted pot of water for the lasagna noodles. Cook the noodles al dente (about 7-8 minutes) - be careful not to overcook. Stir them occasionally to prevent sticking. Once they're done, drain them (do not rinse). When they're cool enough to handle after a couple of minutes, lay them out individually on a sheet of parchment paper so they don't stick together.
To assemble the lasagna: spread about 1 cup of the meat sauce evenly on the bottom of a deep 9x13 casserole dish. Add 4 noodles to the bottom of the casserole dish (they will overlap a bit). Spread half of the cheese mixture over top, and then spoon about 1/3 of the meat sauce on top of that. Add another 4 noodles, the rest of the cheese mixture, another 1/3 of the meat sauce, and then the final layer of 4 noodles. Spoon on the rest of the meat sauce, then top with the remaining 2 cups of mozzarella cheese.
Cover with foil and then bake for 30 minutes. Take the foil off and continue baking for another 20 minutes. Optional: broil for a few minutes to make the cheese extra browned (watch it closely so it doesn't burn!).
Let the lasagna sit for 15 minutes. This is important as it helps it set. Sprinkle with chopped parsley and basil if using. Cut into squares (I use my chef's knife and then a spatula to scoop it out) and enjoy! Season with extra salt & pepper if needed.