Put potatoes in a large pot, cover with water, and boil until tender. Mash potatoes in a bowl
Add salt, milk, 2 eggs, ham or bacon (if desired), cheddar, and scallions to the bowl with the potatoes. Stir to combine and add more milk as needed until it looks like waffle batter (thick but not too thick)
Cook in a waffle iron according to the directions of your waffle iron
While waffle is cooking, make some fried eggs (over-medium works nicely)
Serve waffle with fried egg on top and sliced avocado if desired