Heat olive oil in a large skillet over medium heat. Add diced onion and minced garlic, stirring until onion turns translucent and garlic becomes fragrant, about 5 minutes.
Add ground beef to the skillet. Increase heat; cook until the beef is browned, breaking up large clumps with a spoon.
Add salt, black pepper, Italian seasoning, and crushed red pepper to the beef mixture. Stir to combine.
Fold in chopped spinach and stir until wilted and mixed well with beef.
Sprinkle grated Parmesan cheese over the mixture and combine until cheese is melted. Serve immediately.
Notes
Store leftovers in an airtight container in the refrigerator for up to 3 days, or freeze for up to 3 months.