1. 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 ¼-inch slices. Set the potatoes aside in a bowl tightly covered with aluminum foil.
2. 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.
