Bring the stock and onions to a boil over high heat in a large saucepan

Add the salt and potatoes

If there isn't enough stock to cover the potatoes, add more

Once the pot returns to a boil, cook for 20 minutes, until the potatoes are tender

Remove the potatoes with a slotted spoon and peel as soon as you can

(If you tear the skin off in large enough pieces, you can toss the pieces of skin with butter and bake them in a hot oven until crisp for a wonderful snack.) Remove the onion from the stock with a slotted spoon

Halve the peeled potatoes lengthwise, then cut into I/4"-thick half circles

Return the potatoes to the stock and let them cool to room temperature

Cooling them in the stock lets them absorb a lot of wonderful flavor

When the potatoes and stock are room temperature, remove the potatoes with a slotted spoon and place in a salad bowl with the celery and sausage

Remove the onions with a slotted spoon, finely chop, and add to the bowl

Place the oil, vinegar, mustard, parsley, thyme, pepper, and 2 tablespoons of the stock into a jar, screw on the lid, and shake well

Pour over the potatoes and toss gently, being careful not to crumble the potatoes

Serve warm or at room temperature.