In batches take cold bee trimmings and place them in a food processor. Pulse until chopped into small even pieces then transfer to cast Iron pot or dutch oven. Repeat until all the beef trimmings are chopped. If you don't have a food processor you can do this with a knife
Once all the trimmings are chopped and in the pot add about a 1/2 cup of water and begin to simmer over medium heat stirring every few minutes.
after about 10-15 minutes the meat will begin to look gray. Continue stirring and making sure the trimmings don't stick to the bottom of the pot.
After about 30-45 minutes the beef trimmings should be dark brown and all of the water should be evaporated. You should see fewer and fewer bubbles as the beef darkens.
Once the beef is dark but not burned use a slotted spoon to scoop out all the beef bits and spread them out on a paper towel. season with kosher salt.
Pour the remaining fat through a fine-mesh strainer and then through a cheesecloth or coffee filter so that the oil is completely clear. Pour into a sanitized mason jar with a lid and and refridgerate.
Notes
Serving size 1 tablespoon. Servings will vary based on fat content from rendering