In a medium saucepan, stir together beef broth, onion powder, garlic powder, kosher salt, black pepper and thyme. Bring to a boil over medium heat, then turn down heat and simmer for 5 minutes.
While broth mixture is simmering, mix together cornstarch and cold water until cornstarch has completely dissolved.
Add cornstarch mixture to broth mixture, and stir until well combined. Bring to a boil then reduce and simmer for another 5 minutes, until broth has thickened.
Stir in butter and cream. Serve immediately.
Notes
Make sure to combine cornstarch into very cold water and mix it until no powder is left, otherwise the gravy will get tiny white lumps. Store in an airtight container in the refrigerator for up to 2 days, or in a freezer bag for up to 2 months.