German Potato Salad

Modified from the Better Homes and Gardens New Cook Book (1970).
Ingredients:
1/2 c. chopped onion
2 tbsps. all purpose flour
2 tbsps. sugar
1 1/2 tsps. salt
1 tsp. celery seed
1/2 c. white vinegar (or part white vinegar, part apple cider vinegar)
6 c. sliced cooked potatoes

Cook onion in olive oil until tender. Blend in flour, sugar, salt, celery seed, and dash pepper; add vinegar and 1 cup water; cook and stir till thickened and bubbly. Add potatoes; heat thoroughly, tossing lightly. Serves 8 to 10.