Cook potatoes unpeeled, only until tender, then drain and peel while still warm.
Slice potatoes and set aside. Cook bacon until crisp, then remove bacon, drain on unglazed paper.
Pour off all but 2 tbsp. of bacon fat, add onion and cook until transparent, not brown.
Add sugar, salt and flour, then blend thoroughly.
Add vinegar and water, cook and stir until smooth and thickened.
Add potato slices and bacon bits, folding ingredients together carefully.
Allow to stand in a warm place so potatoes will absorb flavor of dressing.
Transfer to a warm serving dish, then garnish with parsley and radishes.