Preheat oven to 375°F (190°C).
If using regular Ronzoni lasagna noodles, cook according to package directions until al dente. Drain and set aside.
In a large skillet, heat olive oil over medium heat. Add chopped onion and sauté for 2–3 minutes, until translucent. Add minced garlic and cook for 30 seconds.
Add ground beef (or turkey) to the skillet and cook until browned, about 5–7 minutes. Stir in marinara sauce, oregano, salt, pepper, and red pepper flakes (optional). Let simmer for 10–15 minutes.
In a medium bowl, combine ricotta cheese, egg, Parmesan, and parsley. Mix until smooth.
Assemble the lasagna in a 9x13-inch baking dish:
Spread 1 cup of the meat sauce on the bottom of the dish.
Layer 3–4 Ronzoni noodles on top of the sauce.
Spread ⅓ of the ricotta mixture over the noodles, followed by a layer of shredded mozzarella.
Repeat the layers two more times (sauce, noodles, ricotta, mozzarella).
Finish with the remaining sauce and mozzarella on top.
Cover with aluminum foil (spray the underside with cooking spray) and bake for 25 minutes. Uncover and bake for an additional 15–20 minutes until the top is golden and bubbly.
Let the lasagna rest for 10–15 minutes before slicing.