Have ready a steamer large enough to accommodate the potatoes. Prepare the pan with water.
Cut the potatoes in eights, cut each in half, then those halves, and half again. Put them into the pan. Put the potatoes on to heat.
While that is happening, cook the bacon until crisp in a skillet, or microwave. Do NOT throw away the fat!
When the potatoes are fork-tender, put them in a bowl with the chopped onions. Crumble the bacon overall.
If you cooked the bacon in a pan, heat the pan on low, then add the vinegar and mustard to the pan, and whisk until well mixed. Then add the mayonnaise and whisk briskly to combine. If you used a microwave oven, put the fat in a saucepan, and heat it as previously described.
Pour the mixture over the potatoes, and toss to combine. Refrigerate until ready to serve.
Just before serving, add the crumbled blue cheese, and chopped parsley.
Notes
Quick notes Try to get German whole grain mustard. Yellow mustard will not work in this, it's too sour. Variations If you want to make this vegetarian, you could use olive oil instead of bacon fat, and add walnuts for crunchiness.
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