Bring a large pot of salted water to a boil. Cook the pasta according to package directions until al dente. Reserve about 1 cup of the pasta water before draining. Set the pasta aside.

While the pasta cooks, heat the olive oil in a large skillet or Dutch oven over medium-high heat. Add the ground beef and chopped onion. Cook, breaking up the beef with a spoon, until the beef is browned and the onion is softened, about 5-7 minutes. Drain off any excess grease.

Stir in the minced garlic, kosher salt, black pepper, dried oregano, and red pepper flakes (if using). Cook for 1 minute until fragrant.

Add the tomato paste and stir it into the beef mixture. Cook for 1-2 minutes to deepen the flavor. Pour in the beef broth to deglaze the pan, scraping up any browned bits from the bottom. Stir in the crushed tomatoes.

Bring the sauce to a simmer, then reduce the heat to low. Let it simmer gently for 10 minutes to allow the flavors to meld, stirring occasionally.

Reduce the heat to the lowest setting. Slowly stir in the heavy cream until fully incorporated. Add the fresh spinach in handfuls, stirring until it wilts into the sauce. Stir in the grated Parmesan cheese until the sauce is smooth and creamy.

Add the drained pasta to the skillet with the sauce. Toss everything together until the pasta is well-coated. If the sauce is too thick, add a splash of the reserved pasta water to reach your desired consistency. Taste and adjust seasoning if needed.

Serve immediately, garnished with fresh parsley and extra Parmesan cheese.