Preheat oven to 400 degrees F.
Arrange boiled potatoes on a baking sheet. Using a meat mallet, smash each potato. Season all with salt, pepper, then drizzle with olive oil. Bake for approximately 30 minutes (or until tender on the inside, crispy on the outside.
While baking, put parsley, cilantro, garlic, Serrano, vinegar, salt, red pepper, and pepper in a food processor and blend until it’s a chunky consistency. Transfer to a mixing bowl and whisk in remaining oil.
Serve potatoes with sauce on top or on the side.