Carefully heat bacon grease in a skillet or small saucepan. Sauté the chopped onion for about three to five minutes, or until tender. 

Add the white vinegar into the pan and stir. Add the sugar, mustard and water. Stir the mixture until well combined. 

Remove from heat. Cool slightly and then pour over salad and serve.

Extra dressing can be stored in the refrigerator for up to 5 days. Simple warm up salad dressing before using.