Warm stockpot over medium-low heat. Add cooking spray.

Add beef fat chunks to stockpot. Fix lid onto stockpot and heat to a slow simmer, stirring every 20-30 minutes. Warm until the tallow has completely rendered, about 3-4 hours. If it starts to boil, reduce heat immediately.

Once the tallow has rendered to a liquid, remove the stockpot from heat. Pour the liquid tallow through the strainer into a heatproof container.

Strain the tallow a second time to remove any further impurities. This time strain the liquid tallow through cheesecloth.

Decant the strained mixture into a mason jar or airtight container. Leave to cool completely before applying an airtight lid and storing in the refrigerator. Once cooled, it will be solid white in color.

Store in refrigerator for up to 3 months