Preheat the waffle iron and set it to medium high.
Mix together mashed potatoes, buttermilk, scallions, cheddar, egg, and butter or bacon fat until combined. Stir together flour, baking powder, baking soda, and salt in a small bowl. Add the flour mixture to the potatoes and mix.
Once the waffle iron is hot, spray or brush some oil on both sides. For a two section waffle iron, add about 1/3 cup of the potato mixture to each section of the waffle iron. What you are looking to do is to make rough waffle circles so you may need to add a little more or a little less depending on the size of the sections in your waffle iron.
Cook until the waffles release from the iron and are golden brown, about 5 minutes, though you may need a minute more or less depending on your iron.
While the waffles are cooking, whisk together all the sour cream sauce ingredients.
To serve, top the waffles with the sour cream buttermilk sauce and scatter some sliced scallions over the top.