Preheat the oven to 400 degrees Fahrenheit. Line a rimmed baking sheet with parchment paper.
Place two wooden skewers (chopsticks or wooden spoon handles) on either side of a potato. Starting where the potato curves down and its bottom meets the cutting board (this will prevent you from cutting off the end of the potato), create slices down the length of the potato, 1/8 inch thick. Use the wooden skewers as a guide so that the knife doesn't go all the way through the potatoes. Repeat with the remaining potatoes and place them on the baking sheet.
6 medium russet potatoes
In a small bowl, whisk together the butter, olive oil, salt, black pepper, and garlic. Brush half the mixture over the potatoes.
3 tablespoons unsalted butter, 2 tablespoons olive oil, 1 1/2 teaspoons kosher salt, dash of black pepper, 3 garlic cloves
Bake for an hour. Halfway through baking, remove the potatoes from the oven. Put a fork under the potato in the middle of it, and gently lift it about two inches off the baking sheet. This will help the potato open up more (repeat with the rest of the potatoes).
Then, gently use a fork to spread the potatoes open from the top. Brush on the remaining butter mixture and return to the oven for the remaining baking time. The potatoes are done when the edges are crisp and the centers are fork-tender and soft.