Step 1

Bring potatoes, 1 cup Diamond Crystal or ½ cup plus 1 Tbsp. Morton kosher salt, and 3 quarts water to a simmer in a medium pot over medium heat; cook until tender, 18–22 minutes.

Step 2

Meanwhile, using a slotted spoon, carefully lower eggs into a large saucepan of boiling water and cook, adjusting heat as needed to maintain a gentle boil, 10 minutes. Transfer eggs to a bowl of ice water and let cool until just slightly warm, about 2 minutes.

Step 3

Remove eggs from ice water. Gently crack all over and peel; discard shells. Cut eggs in half lengthwise and scoop out yolks into a medium bowl. Coarsely chop egg whites; set aside. Mash yolks with a fork until smooth. Add mayonnaise, relish, both mustards, and remaining 2 Tbsp. Diamond Crystal or 1 Tbsp. Morton kosher salt; mix well. Fold bell pepper, red onion, and celery into dressing.

Step 4

Drain potatoes; let cool 5 minutes. Peel with a paring knife; cut into ½" pieces. Add to dressing and gently toss; fold in reserved egg whites. Cover and chill at least 1 hour and up to 3 days.

Step 5

Sprinkle with seasoning before serving.