First, slice a whole pile of onions. I normally cut them in half, and then slice into half moons. Note that my pot was almost full at the beginning. (I don't remember exactley but about 8 of those great big softball sized sweet onions would do)
Start a soup pot over medium heat. Add a little oil to the pan, and start cooking.
Stir occasionally. We're trying to both soften the onions and release a lot of liquid.
Keep stirring every few minutes. Note the brown bits I just stirred up. Every time you stir make sure to stir every bit of the bottom of the pot.
Eventually it will become a dark caramel color. Keep cooking, and keep stirring. Once it gets to this stage I add a little liquid each time I stir (apple cider, water, or chicken broth). We still want to get the color a bit darker, but don't want to burn the onions.
Keep adding liquid and stirring:
Once you are satisfied with the deep brown color, add liquid to make broth. I normally use about half apple cider, half chicken broth. Make sure to add some salt if you use apple cider. The flavor of the onions will be the star of this soup, so if you add other spices make sure they are not scene stealers. I added about 8 cups total liquid.