Boil the tortellini: Bring a large pot of salted water to a boil.
Cook tortellini according to package directions until just al dente. Reserve 1/2 cup pasta water, then drain.
Brown the beef: While the pasta cooks, heat a large skillet over medium-high. Add beef and break it up.
Season with salt and pepper. Cook until well browned and no longer pink, about 6–8 minutes. Drain excess fat if needed.
Sauté aromatics: Push the beef to one side.
If the pan looks dry, add olive oil. Add onion and cook 3–4 minutes until softened. Stir in garlic for 30 seconds until fragrant.
Season the base: Sprinkle in Italian seasoning and red pepper flakes.
Toast the spices for 30 seconds to wake up their flavor.
Build the sauce: Pour in marinara and beef broth. Stir to combine, scraping up any browned bits from the pan. Simmer 3–4 minutes to meld flavors.
Add cream: Lower heat and stir in the cream.
Simmer gently 1–2 minutes. Taste and adjust salt and pepper.
Add greens and cheese: Stir in spinach until wilted, about 1 minute. Mix in Parmesan until melted and the sauce looks glossy.
Combine with tortellini: Add cooked tortellini to the skillet and toss to coat.
If the sauce is too thick, loosen with a splash of reserved pasta water. If it’s thin, simmer 1–2 minutes more.
Finish and serve: Top with more Parmesan and fresh basil or parsley. Serve hot.