Heat a Dutch oven or large pot over medium-high heat. Add the ground beef and let it brown without stirring for 5–6 minutes to develop color, then break it up slightly.
Add the chopped onion and continue cooking, stirring and breaking the beef into small pieces, until the onion is softened, about 5 minutes. Spoon out excess fat if there is a lot.
Stir in the minced garlic, crushed red pepper flakes (if using), and Italian seasoning, then add the orzo and cook for about 1 minute to toast slightly.
Pour in the tomato sauce, beef broth, heavy cream, and Worcestershire sauce. Bring to a gentle simmer, then reduce heat if needed so the mixture bubbles gently.
Cook uncovered, stirring frequently so the orzo doesn't stick, until the pasta is tender and most of the liquid is absorbed, about 10 minutes.
Remove the pot from the heat, stir in the grated Parmesan and the baby spinach until the spinach wilts and the sauce thickens, about 3–5 minutes. Season with salt and pepper to taste and serve immediately.
