Preheat the oven to 200°F
Cut the fat into cubes, about 1/2 - 1" in size
Place the fat cubes in the dutch oven or pot, and add the kosher salt and water.
Place the dutch oven in the center of the oven and cook for about an hour at 200°F. Don’t increase the oven temperature, you only want the fat to render.
After an hour, check that the fat still looks good (no browning), and continue cooking for another 12 hours (or overnight).
It’s now time to strain the fat. Place a small piece of cheesecloth underneath the lid of the fat separator. Use a ladle to scoop the fat out of the pot onto the strainer lid.
When you’ve removed as much fat as you can using the ladle, carefully tip the pot to transfer all the remaining contents into the fat separator. Remove the solid fat pieces on top of the strainer and discard.
Allow to cool and separate at room temperature for about 15 minutes. As it cools, the now liquid tallow will rise to the top, and any remaining water and impurities will fall to the bottom. To separate the tallow (on top) from all the impurities, working over a small bowl release the liquid on the bottom of the fat separator until you start to see the clear liquid tallow.
Once all the impurities are removed, drain the remaining liquid tallow into the mason jar or storage container. Close the jar and store it in the refrigerator.
Cool in the refrigerator for at least 2 hours. Once it appears white and solid, it's ready to use!