Preheat and prep: Place a large, deep skillet over medium-high heat. If using very lean beef, add olive oil to the pan to prevent sticking.
Brown the beef: Add the ground beef and break it up with a spatula. Cook 4–6 minutes until browned and no longer pink.
If there’s excess grease, drain it off.
Build the flavor base: Add the diced onion and cook 3 minutes until softened. Stir in garlic, salt, pepper, Italian seasoning, and red pepper flakes. Cook 30–60 seconds until fragrant.
Add sauce and liquid: Pour in the marinara and 1/2 cup beef broth.
Stir well, scraping up any browned bits from the pan. Bring to a gentle simmer.
Add tortellini: Stir in the tortellini, ensuring it’s mostly submerged. If the sauce looks too thick, splash in a bit more broth or water.
Cover and simmer over medium heat for 5–7 minutes, stirring once or twice, until the tortellini is tender.
Finish the skillet: Reduce heat to low. Fold in the spinach and cook 1–2 minutes until wilted. Stir in Parmesan.
If using mozzarella, scatter it over the top, cover, and let it melt for 1–2 minutes.
Taste and adjust: Taste the sauce. Add a pinch more salt, pepper, or red pepper flakes if needed. The sauce should be savory and slightly bright.
Serve: Spoon into bowls and top with extra Parmesan and fresh herbs.
Serve hot.