Step 1

Place potatoes in medium pot and cover with cold water. Bring to a boil, add ½ tablespoon salt, reduce heat and simmer until just tender, 12 to 15 minutes.

Step 2

Meanwhile, toss onion with vinegar and ½ teaspoon each salt and pepper and let sit, tossing occasionally.

Step 3

Drain potatoes and run under cold water to cool. Drain well and pat dry, then halve any that are large.

Step 4

Whisk oil and mustard into vinegar mixture. Add potatoes and toss to coat. Fold in peas and dill.