Bring a large pot of salted water to a boil. Add the medium pasta shells and cook according to package instructions (8-10 minutes). Drain and set aside.

In a skillet, heat 1 tablespoon of oil over medium heat. Sauté the chopped onions for 3-4 minutes until translucent. Add minced garlic and cook for 30 seconds.

Add ground beef, cooking for 5-7 minutes until browned. Stir occasionally.

Pour in the tomato sauce and cream, stir, and add Italian seasoning and mustard. Simmer for 5-7 minutes until thickened.

Fold the drained pasta shells into the skillet with the creamy beef sauce until well-coated.

If using cheese, stir in until melted. Adjust seasoning with salt and pepper as needed. Serve hot.