Melt butter in a saucepan over medium heat. Add garlic and rosemary and saute for 1-2 minutes or until fragrant.
Add heavy cream and bring to a gentle boil over medium heat, whisking constantly.
Whisk in parmesan cheese and garlic salt.
Reduce heat to low. Add cornstarch and whisk over low heat until thickened.
Notes
This sauce is excellent over fish and/or fresh vegetables. Can also be served over other meats like chicken and steak.
Store leftovers in an air-tight container or mason jar in the refrigerator for 3-4 days.
While using cornstarch is not a necessary step as you can simply continue to simmer the sauce until the cream cooks down and thickens, you will lose volume. Using cornstarch thickens it quickly and allows you to keep the volume.