Fill a small sauce pan about 2 inches up the side with water. Add vinegar and 2 tsp kosher salt.
Heat water to just under boiling, to the point where tiny bubbles collect on the bottom of the pan.
While water is heating, crack each egg into an individual ramekin or small custard cup. Careful not to break the yolk.
When water is ready, swirl it around rapidly with a spoon to create a whirlpool, then gently slide each egg into the middle of the whirlpool. (Note: You'll need a bigger saucepan if you're poaching more than 1 or 2 eggs.) The whites will swirl around a bit but the whirlpool should contain it. RESIST the urge to touch or coax the eggs.
Immediately turn off the heat, cover your pan and set a timer for 5 minutes. DO NOT LIFT THE LID OR STIR THE EGGS. Trust the process and in 5 minutes you'll have perfect poached eggs.
After 5 minutes remove the eggs from the water with a slotted spoon and slide each one onto the top of the mashed potato mixture in the cups.
Top eggs with remaining creme fraiche and chopped scallions.