Brown the meat: Heat the olive oil in a large pot over medium heat and cook the onion for 4-5 minutes until soft. Add the ground beef and Italian sausage and cook for 8-10 minutes, breaking the meat apart, until no pink remains and both reach 160°F. Drain off the grease.

Bloom the garlic and paste: Stir in the garlic and tomato paste and cook for 1 minute, until the paste darkens and smells sweet instead of raw.

Simmer the sauce: Add the crushed tomatoes, tomato sauce, sugar, Italian seasoning, salt, black pepper, and red pepper flakes if using. Bring to a low simmer and cook uncovered for 30 minutes, stirring now and then, until the sauce is thick enough to mound on a spoon.

Boil the noodles: While the sauce simmers, cook the lasagna noodles in a separate pot of salted water for 1-2 minutes less than the package says. Drain and lay them flat on a sheet pan or a lightly oiled counter so they don't stick together.

Mix the ricotta filling: Stir together the ricotta, egg, parmesan, parsley, and 1/2 teaspoon salt in a bowl until smooth. The egg is what keeps the filling from running out the sides once it heats up.

Layer the pan: Spread 1 cup of sauce across the bottom of a 9x13 baking dish. Add 4 noodles, half the ricotta mixture, a third of the remaining sauce, and 1 cup of mozzarella. Repeat with 4 more noodles, the rest of the ricotta, another third of the sauce, and 1 cup of mozzarella. Finish with the last 4 noodles, the remaining sauce, and the last 2 cups of mozzarella.

Bake covered, then uncovered: Heat the oven to 375°F. Cover the dish with foil (spray the underside so it doesn't stick to the cheese) and bake for 25 minutes. Remove the foil and bake 20-25 minutes more, until the edges bubble and the top is golden and spotted brown.

Rest and slice: Let the lasagna sit for 15 minutes before cutting. Scatter fresh basil over the top and serve.