Preheat oven to 425°F.
Place potatoes on a cutting board, slice potatoes widthwise into thin slices about 1⁄4 inch apart, leaving a 1/2 inch at the bottom to keep the potato intact.
In a small bowl, mix melted butter and oil together. Place potatoes in an 8 × 8-inch baking dish, season with salt and pepper, brush over half the butter/oil mixture.
Bake in the oven for 30 minutes. Remove from oven and brush with remaining butter mixture. Return to oven and cook until tender, about 30 minutes.
Serve roasted hasselback potatoes alongside homemade ranch sauce for dipping.
