Melt butter in a saucepan over medium-low heat. Whisk in flour until fragrant, 10 to 12 minutes.
Gradually whisk in cold stock. Sprinkle in sage and whisk. Bring gravy to a boil. Reduce heat to low boil. Cook and stir until thick enough to coat the back of a spoon, about 5-10 minutes, whisking regularly.
Stir in heavy cream, and season with salt, white pepper, and cayenne pepper.
After you reduce the heat to a low boil, make sure you whisk regularly while the gravy is thickening, otherwise the bottom can burn.
Butter: You can sub an equal amount of drippings in place of the butter.
Storage: Can be stored in an airtight container in the fridge for 2-3 days, or in the freezer for about 3-4 months.
Make it Gluten-Free: Use gluten-free all-purpose flour or cassava flour 1:1 instead of traditional flour.