Measure out the ingredients -for a gluten free sauce, use a gluten free flour mix. I like the Bob's Red Mill All Purpose Baking Flour. It's a versatile flour blend that works well in many recipes. If you can't find it, use another gluten free flour blend

Mix the flour, sugar, eggs and dry mustard together in a heavy bottom sauce pan

Add in the vinegar and milk and stir to combine. I know it sounds odd to mix vinegar and milk, but trust me it works beautifully

Bring the mixture to a boil over medium heat - whisking often to remove any lumps

Boil the mixture, stirring often to ensure it doesn't burn, until the custard becomes thick (like hot pudding) - about 5 to 10 minutes

Let the sauce cool to room temperature and refrigerate if not using immediately

Serve the sauce with ham at room temperature or cold. Refrigerate the leftover sauce. It's fabulous on sandwiches