In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes.
Add the minced garlic and ground beef to the skillet. Cook until the beef is browned, breaking it apart with a spoon, about 5-7 minutes. Drain excess fat if necessary.
Stir in the orzo, diced tomatoes (with juice), beef broth, Italian seasoning, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to low, cover the skillet, and simmer for about 10-12 minutes, or until the orzo is tender and has absorbed most of the liquid. Stir occasionally to prevent sticking.
Once the orzo is cooked, sprinkle the shredded mozzarella cheese over the top. Cover the skillet again and let it sit for 2-3 minutes until the cheese is melted.
Garnish with fresh basil leaves if desired and serve hot.