Rinse and scrub potatoes and slice into 1/4-inch slices. Add to a large saucepan and cover with cold water. Add salt and bring to a boil. Once boiling, reduce heat to medium/high and continue cooking for 10-13 minutes or until the potatoes are easily pierced with a knife. SEE NOTES ABOVE ON COOKING POTATOES.
While potatoes are cooking, prepare vegetables and dressing. Add mustard, garlic, salt, pepper, rice vinegar, and apple cider vinegar to a mixing bowl and whisk to combine. Whisk olive oil in a slow stream to emulsify the oil and vinegar.
Once the potatoes have finished cooking, drain and rinse with cold water. Once they’re dry, add to a large serving bowl. Season with salt and black pepper and 1 tablespoon apple cider vinegar.
Add corn, scallions and dill.
Pour in the dressing and toss to combine. Taste the potato salad and adjust salt and pepper. Chill for 1 hour or longer to let the potatoes soak in more of the flavor of the dressing as it sits.
Serve chilled, or at room temperature. Store leftovers covered in the refrigerator up to 4 days. Do not freeze.