Using the same pan where you seared the beef tenderloin, add the red onions and let them cook and soften for 1 to 2 minutes.
Add the rest of the ingredients, then bring the blueberry mix to a low simmer with occasional stirring. This should take about 5 to 10 minutes. Don’t wait for the syrup to thicken, as it’ll get thicker as it cools down.
Remove the syrup from heat and let it cool for 5 minutes, then remove the depleted thyme sprigs
Ladle over the reverse sear beef tenderloins on a serving platter.