Add 1 tablespoon of butter to a heavy bottomed pan, and sautee the onions for about 20-25 minutes, over low-medium heat until they are soft and nicely browned. Take your time here and let them get nice color, it will result in great flavor later on.
Add the garlic, and sautee for 1-2 minutes more.
Add the rest of the butter to the pan, and while it is melting, slowly add the flour, stirring as you go to avoid lumps.
Cook the roux for about 10-15 minutes, you're going to want to stir often, until it's a rich amber brown color, but not burnt!
Slowly stir in the apple cider, Worcestershire sauce, and salt, and continue stirring until everything is smooth.
Add the chicken stock and the sage, stir until everything is evenly incorporated, and gently bring the gravy to a simmer.
Simmer for about 10 minutes or until thickened. This gravy isn’t super thick, but it should definitely coat the back of a spoon.
At this point, you have a couple options. If you are serving right away, remove the sage leaves (if you left them whole), and you can either serve the gravy as is, or strain first if you want a very smooth gravy.
If you are making this gravy ahead, refrigerate and then re-heat and strain when you are ready to eat. That way, all the flavors can meld and infuse overnight.