Cook onion in a few tablespoons of butter. Add ground beef and season with salt and pepper. Cook and crumble until browned.
In a saucepan, melt a few tablespoons of butter and cook garlic for 30 seconds to a minute. Add cream.
Mix cornstarch with equal parts water and add to saucepan. Bring to a boil, then reduce and cook until thickened.
Add 2/3 to 3/4 cup of Gruyere and stir until melted.
Meanwhile, cook pasta; then drain. Place back in pot and add ground beef and sauce.
Stir in tomatoes, parsley, and rest of cheese.