In a small, dry skillet, add the almonds and hazelnut pieces over medium heat. Shaking the skillet often, lightly toast the nuts until fragrant, 4 to 5 minutes.
In a food processor or small blender, combine the ingredients for the romesco sauce, including the toasted nuts. Puree until the sauce is fairly well smooth. Taste and add salt and pepper as desired. Measure out 1/2 cup and save the remaining sauce for another use.
Bring a pot of water to a boil. Quarter (or halve, depending on the size) the potatoes and add to the water, cooking for 3-4 minutes. The potatoes should just be tender and have a good firmness to them. Drain and place in a roasting pan along with the 1/2 cup romesco sauce.
Stir until the potatoes are well coated. Bake for 25 to 30 minutes. Potatoes should be starting to brown and be tender. Remove and sprinkle with chives if using.