How to Make It
In a large pot of boiling salted water, cook the potatoes over high heat for 5 minutes. Drain and let cool slightly, then halve the potatoes lengthwise.
In a large saucepan, combine the tomatoes, garlic, shallot, ancho chile powder, pimentón and water and bring to a boil. Simmer over moderate heat for 25 minutes, then transfer to a food processor. Add the pumpkin seeds and 1/4 cup of the oil and puree. Season the romesco with salt.
Meanwhile, preheat a large cast-iron griddle on the grill. In a bowl, toss the potatoes with the remaining 1/4 cup of oil and season with salt. When the griddle is very hot, add half of the potatoes in a single layer, cut sides down. Cook them over high heat until browned, 10 minutes. Turn the potatoes and cook until the skins are browned in spots, 5 minutes longer. Transfer the potatoes to a platter. Cook the remaining potatoes. Alternatively, preheat the oven to 425°. Arrange the potatoes cut side down on 2 baking sheets. Roast for about 25 minutes, switching the position of the pans halfway through, until the potatoes are browned.
Drizzle the potatoes with some of the romesco sauce and serve, passing the remaining sauce at the table.