6 med.-sized boiling potatoes (about 2 lbs.), scrubbed but not peeled
1 c. finely chopped onions
2/3 c. chicken stock, fresh or canned
1/3 c. olive oil
1 tbsp. white wine vinegar
2 tsp. prepared Dusseldorf-style mustard, or substitute 2 tsp. other hot, prepared mustard
2 tsp. salt
1 tsp. freshly ground black pepper
1 tbsp. fresh lemon juice

Drop the unpeeled potatoes into enough lightly salted boiling water to cover them completely. Boil them briskly until they show only the slightest resistance when pierced with the point of a small, sharp knife. Be careful not to let them overcook or they will fall apart when sliced. Drain the potatoes in a colander, then peel and cut them into 1/4-inch slices. Set the potatoes aside in a bowl tightly covered with aluminum foil.

In a heavy 2 to 3-quart saucepan, combine the chopped onions, stock, oil, vinegar, prepared mustard, salt and pepper. Bring to a boil over high heat, stirring occasionally. Reduce the heat to low and simmer uncovered for 5 minutes. Remove the pan from the heat and stir in the lemon juice.

Pour the sauce over the potato slices, turning them about with a spatula to coat them evenly. Let the potatoes cool to room temperature, then taste for seasoning and serve.