Once the bacon is cooked, keep 2 - 3 tablespoons fat in a small skillet, over medium-low heat add the flour and cook, stirring constantly, until it has browned slightly and thickened. (this is like making a roux, except that's generally one part fat to one part flour, we want this looser, so we are doing one part fat to one-third part flour); once the flour is browned and cooked, about 3 - 4 minutes, add the vinegar, stir until thickened. If it becomes too thick, add a bit more vinegar. Now comes the sugar, if you added more vinegar, you might need to add more sugar. So I suggest you take a bit of a taste, if your face puckers, add a large pinch of sugar if it doesn't add a small pinch. Once it's the perfect non-pucker, sweet, sour, bacon tasting goodness it's supposed to be, pour it over your potatoes, onions, pickles. Stir to combine. Taste and adjust salt and pepper. Now gently stir in the parsley and serve.