I dice potatoes and place into water, dice ham and onions and put with them. When potatoes are almost done I make up the roux for the sauce. 

Roux ~ melt 7 tbsp of butter, then add the flour to it, when combined slowly add the milk. When mixed together then add the cheese.

When all complete, add the sauce to the potatoes, ham and onions. I also drain the water or at least half before putting the sauce in, then I can add water if it needs to be thinner.