Heat the olive oil in a large pot over medium-high heat.
Add the ground beef and cook for 7–10 minutes, breaking it apart as it browns. Cook until no longer pink.
Add the garlic and cook for about 1 minute, stirring frequently.
Stir in the Italian seasoning and marinara sauce.
Fill the empty marinara jar with water, secure the lid, shake well, and pour the water into the pot.
Add the rotini and stir everything together.
Bring the mixture to a boil, then reduce the heat to a simmer. Cover and cook for 12–15 minutes, or until the pasta is tender. Stir frequently to prevent the pasta from sticking to the bottom.
For this easy one pot pasta with ground beef, gently stir in the fresh spinach and ½ cup of Parmesan.
Continue stirring until the spinach wilts and the Parmesan melts into the sauce.
Sprinkle the remaining ¼ cup Parmesan over the top and serve warm.