Step 1

Combine both types of potatoes in a large pot and fill with enough water to cover them. Bring to a boil over medium-high heat, then lower the heat to maintain a gentle boil. Cook for 10-15 minutes or until the potatoes are tender but still hold their shape. Drain the potatoes and allow to cool slightly.

Step 2

In a separate bowl, mix together the mayonnaise, mustard, sugar, vinegar, and salt until smooth.

Step 3

In a large mixing bowl, gently combine the cooked potatoes, celery, onion, and sweet pickle relish.

Step 4

Pour the dressing over the potato mixture and gently stir to coat all the ingredients evenly.

Step 5

Cover the bowl with plastic wrap and refrigerate the potato salad for at least 1 hour, or overnight if possible.

Step 6

Before serving, give the salad a gentle stir to redistribute the dressing and flavors.