Preheat oven to 425 degrees. Carefully slice the potatoes Hasselback style, being careful not to slice all the way through so the potatoes remain in-tact.
Gently brush the potatoes with olive oil then sprinkle with salt and pepper before roasting until golden brown, about 15-20 minutes.
While the potatoes are roasting, whisk together the tahini sauce: tahini through warm water. If the sauce is too thick, add more warm water working 1 tablespoon at a time. Check for seasoning and add more salt or pepper to taste if needed.
To serve, drizzle the potatoes with the tahini sauce or serve it on the side as a dipping sauce.