While the potatoes are roasting, make the romesco sauce. Add all the ingredients into a jar. Pulse all the sauce ingredients, except the olive oil, together with an immersion blender until it reaches the desired texture. Add the olive oil; process until well incorporated. Season with salt and pepper. Taste and adjust seasoning, if desired.
Alternatively, use a food processor or blender to make the sauce.
To crisp up the potatoes even more, flip them after the skin starts to brown and bake them for another 10-15 minutes with the cut side facing upwards.