Toast mustard seeds in a small skillet over medium-low heat, swirling pan occasionally until fragrant and just beginning to pop (3-4 minutes).
Stir 1/4 cup sugar and 1/4 cup water together in a small bowl and pour into the skillet. Stir well and cook until the mustard seeds are swollen, the sugar dissolves and the liquid is reduced and syrupy (2-3 minutes).
Remove from heat and let cool to room temperature. (You can store in an airtight container for up to 6 weeks.)
Cover potatoes in water with the salt and cook until the water dissolves and potatoes are dusty and wrinkled.
Serve with a dollop of crème fraiche, mustard caviar and a showering of shaved scallions and bacon bits.