Using a cast iron skillet on medium low heat, add butter and chopped onion to carmelize, 10 to 15 minutes, being careful not to burn them. Remove onions from pan when done, and put skillet back on fire, adding bacon grease and flour.

Using small whisk, mix thoroughly for a couple of minutes. This will result in a simple roux. Remove most of the roux from skillet and add beef stock. Bring to a boil and whisk in the roux until you reach your desired consistency.

Turn off the heat now and add onions back to the gravy. Season with salt, pepper, worcestershire, and horseradish.