Melt the butter in a large skillet over medium heat. Add the onions and salt. Turn or stir the onions every few minutes until they reach a nice golden brown color. If the onions are browning too fast reduce the heat a bit and if they stick to the bottom of the pan deglaze them with a tablespoon of red wine vinegar, wine, or balsamic vinegar.
Once the onions are browned and caramelized stir in the beef broth (reserving 1/4 cup to mix with the cornstarch), Worcestershire Sauce, and Dijon mustard. Stir to combine and bring to a low boil.
In a small bowl whisk together the reserved beef broth and cornstarch. Slowly add the cornstarch mixture to the gravy whisking to combine. Boil just until thickened.
Reduce heat to low and season with salt and pepper to taste. Whisk in the the fresh thyme. For best results serve promptly.
NOTES
Caramelizing onions is not difficult but it can be time consuming so plan appropriately. I can take up 45 minutes to caramelize onions. Of course there are different degrees of caramelization and for this particular gravy you don’t have to go the full enchilada.
Slice those onions thin. They will be so much easier to caramelize if they are thin.
Stir the onions but not to frequently as they must have some surface time with the hot skillet in order to caramelize.
If the onions stick to the pan add a little more butter or olive oil. Or simply deglaze by adding just a touch of wine, balsamic vinegar, or apple cider. If it is at the end of the caramelization process simple deglaze with the beef broth.
Store the gravy in an airtight container in the fridge for up to 4 days. Reheat in the microwave at a reduced power or on the stovetop over low heat.
Freeze for up to 3 months in freezer zipper bag or freeze container. Keep in mind though that sometimes cornstarch gravy thins after being frozen.
