put onion and eggs in blender/ninja first with salt and pepper. Blend high just until liquid and stop.
add potatoes a little at a time and do a quick blend just to mash them in the mix. Do this until all potatoes are added and blended together as a mash, adding 2-3 Tablespoons of the milk and the sugar now. Do NOT liquify. You want it thick.
Heat enough oil to a medium high heat in a large frying pay (12 inches or so), large enough so the heat distributes evenly and browns the pancakes pretty evenly. Make sure it's hot before putting batter in oil or the batter will just absorb the oil and you will have greasy pancakes.
Pour mixture into large bowl (with handle if you have one) and gradually add enough flour to thicken but not too thick. You want the pancakes to pour out of the bowl and stay together once they hit the hot oil. Try a little one, see how it goes, then adjust the batter if need be. If the batter breaks up, add more flour. If too thick, add some milk. You do want them to be mostly potato, so you don't want to make the batter cake like. If you don't have a pour spout bowl, use a ladle to pour batter into frying pan. Be sure when pouring not to drop it from high or you will splash the hot oil and burn yourself. Lower the ladle to the top of the oil. The batter should sizzle well once put in oil.
You should be able to fit about 5-6 pancakes in a 12 inch pan. Let them fry about a minute, then use a fork to move them around to release them from the pan. If they stick, use a spatula to loosen them, then rotate them in a circle so they fry evenly on all edges of one side.
Depending on if you like thin or thicker, more moist inside pancakes, determine the depth of the batter when you pour it.
Watch for the edges browning a bit, then lift with your fork to look underneath to see if they are nicely browned and crisp. Turn over at this point and finish frying on the flip side.
Remove to either a rack to drip excess oil or onto paper towels. Do not stack them until they have stopped dripping or the ones on the bottom will be greasy.
We like them served with ketchup but some people prefer syrup. If you want to use syrup, I would skip the onion. I like to pour ketchup over them before cutting them up on my plate, but you can use the ketchup as a dip if you like and just pick up the whole pancake and dip them.
These are delicious hot or cold, so make plenty of them. The average adult will eat about 6-8 of them to a meal.