How to Make It
Place potatoes, water, and 2 tablespoons of the salt in a large saucepan; bring to a boil over high. Reduce heat to medium-high; simmer, uncovered, until potatoes are fork-tender, about 8 minutes. Drain.
While potatoes simmer, cook bacon in a large skillet over medium, stirring occasionally, until crisp, about 10 minutes.
Transfer bacon to a plate using a slotted spoon; reserve drippings in skillet. Add onion and scallions to skillet; cook over medium, stirring often, until tender-crisp, about 5 minutes. Remove from heat; whisk in vinegar, mustard, sour cream, and sugar. Add hot potatoes; toss gently to coat.
Transfer potato mixture to a large bowl; stir in dill, parsley, pepper, and remaining 1 1/4 teaspoons salt. Serve warm or at room temperature.