Combine softened butter, olive oil, salt, pepper, and green onions, if desired, in a small bowl. Whisk until well combined.
Slice potatoes almost all the way through at 1/8 inch intervals. (Hint: chopsticks on either side of the potato keep the knife from going all the way through.
Using a pastry brush or spoon, brush sliced potatoes with butter mixture, fanning potatoes out so that butter mixture seeps between slices.