Preheat oven to 250º

Peel potatoes and coarsely grate by hand, transferring to a large bowl of cold water as grated.
Soak potatoes for 1 to 2 minutes after last batch is grated. Then drain well in a collander.

Spread grated potatoes and onion on a clean kitchen towel and roll up like a jelly roll. Twist towel tightly to wring out as much liquid as possible.

Place in mixing bowl and add egg and salt.

Heat a cast iron skillet over medium heat. When skillet is hot:

Add ¼ tsp of olive oil and a slice of butter to the hot pan and immediately add 2 tablespoons of mixture per latke and spread into three inch circles with a fork.

Cook until undersides are browned, about 5 minutes. Flip and brown other sides.
Transfer to paper towels and season with salt.

Add oil and butter as needed for each batch of latkes. Keep latkes warm on a wire rack set in a shallow pan in oven.