Preheat oven to 450ºF. Line a large rimmed baking sheet with parchment paper or foil.Cut each potato into 1/8-inch slices, taking care not the slice the potato all the way through the bottom. As you slice, leave a ½-inch base at the bottom of each potato to hold the potatoes together.Set potatoes on the baking sheet and drizzle the tops with olive oil. Roll potatoes around to coat in oil, then turn them cut-side-up. Squeeze the sides of each potato to open the cuts in the potatoes so the oil works its way down in between each slice.Sprinkle tops generously with salt and pepper. Bake for 45–60 minutes (depending on the size of the potatoes) until slices are crispy and slightly separated, and the base of each potato is soft.Meanwhile, place cilantro, mayonnaise, water, lemon juice, garlic, jalapeno, salt, and ground black pepper in the bowl of a food processor. Puree until very smooth.Serve the hasselback potatoes hot, drizzled with Jalapeno Cilantro Sauce!