Add all ingredients to a large pot over medium heat and whisk well until combined.
Bring the mixture to a boil, then reduce heat to medium and simmer for 15 minutes. Whisk often while cooking, making sure to scrape down the sides of the pot as it cooks down.
Remove from heat and let cool at room temperature for 30 minutes before transferring to a sealed container to refrigerate for 4-6 hours or overnight.