This was a staple in Grandma's kitchen.
3/4 c sugar
1 c vinegar
1 and 1/2 c water
2 tbsp flour
1 tbsp mustard
1 tsp salt
3 eggs, beaten
Combine ingredients and cook until thick in double-boiler over medium heat.
It will keep in the fridge for a month.
I use it in potato salad and cole slaw, and even on tossed salads.