Heat 2 tablespoons of bacon fat over medium-low heat. Sweat the yellow onion, ham hock, sage, black pepper, and crushed red pepper until the onion is translucent.
Add cream and milk and steep over medium-low heat, careful not to boil. When the cream is hot, turn off the heat and let it rest for 30 minutes.
In a cast iron skillet large enough to hold the gravy, make a roux by melting the remaining bacon fat and whisking in the flour until smooth. Ladle the milk mixture into the roux over medium heat. Whisk constantly until smooth before adding more hot liquid.
After all the liquid has been added and thickened, strain it into a clean pan that has a lid so you can keep it warm. Season generously with salt and ground black pepper to taste. Serve hot!