In a large skillet over medium heat, melt the butter then saute for five minutes ham, onions and pineapple.
In bowl, combine juice, vinegar, sugar, mustard and cornstarch. Stir well and pour into skillet. Stir together well, waiting about five minutes for sauce to thicken. Cool and put into large freezer bag or container.
On serving day
On cooking day, reheat in microwave or stovetop. Adjust sauce consistency with cornstarch or water.
Notes
I also freeze leftover ham for recipes like this that call for cooked ham.