Brown beef and onion in a deep 12-inch saute pan over medium-high heat for 7 to 8 minutes until the meat reaches 160°F. Break it into small crumbles and drain excess fat.
Stir in garlic, tomato paste and Italian seasoning for 1 minute. Add tomato sauce, broth, shells, salt and pepper and bring to a simmer.
Cook uncovered over medium-low heat for 11 to 14 minutes, stirring every 2 minutes so shells do not nest together. Add reserved water a little at a time if the sauce dries before the shells are tender.
Stir in cream and simmer gently for 1 minute. Remove from heat and fold in Parmesan until smooth.
Rest for 2 minutes and serve while the sauce still coats the shells loosely.