Mix together the potatoes and, onions. Working in batches and using a blender, puree the potatoes, onions, and just enough of the egg mixture so the mixture is liquid enough to blend until fine but not pasty. Remove the mixture from the blender, scraping into a large bowl. Repeat until all the potatoes and onions have been pureed. 

Fold in the matzo meal, baking powder, sour salt, and salt and pepper. The mixture should be firm enough to just hold its shape but still soft and moist. 

When ready to fry, heat about 2 inches of oil in a wide heavy pan, such as cast iron or stainless steel. To test, using a large kitchen spoon, drop a spoonful of the mixture into the oil. If it holds together, proceed. If not, add  more matzo meal, 1 tablespoon at a time without overcrowding the pan. 

Continue frying the mixture in batches in the hot oil  until crispy and reddish-brown on one side, 3 to 4 minutes adding more oil as needed. Using a slotted spoon or spatula, turn the latkes oven taking care to avoid oil splatters, and fry until browned. 

Scoop the latkes from the oil, drain on a wire rack and keep warm in a 200°F. oven (if they last that long), until all the latkes have been fried. Serve immediately accompanied by cold applesauce and sour cream.

Tips:

Originally, I used starchy Idaho potatoes, but I now prefer the buttery, slightly earthy large golden potatoes—the thicker and rougher the skin, the starchier the potato will be. Small golden potatoes with a thin skin will be too watery. Idaho (russet) potatoes are always an option. 

Using matzo meal, made from ground baked matzos instead of raw flour means the latkes won’t be pasty.

To test the if the oil is hot enough, dip the handle of a wooden spoon or the end of a bamboo skewer into the oil. When ready, the oil will bubble vigorously.