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Preheat the oven to 400°. On 2 large rimmed baking sheets, toss the potatoes with the rosemary and 1/2 cup of the olive oil. Season with salt and pepper and roast for 35 minutes, turning once or twice, until tender and browned. Discard the rosemary sprigs. Reduce the oven temperature to 350°.
Crack half of the eggs into a large bowl. Season with 1 1/2 teaspoons salt and 1/2 teaspoon pepper and beat well. Pour 2 tablespoons of the olive oil into a 10-inch cast-iron skillet. Add half of the potatoes and cook over moderately high heat until they are warmed through. Pour the beaten eggs over the potatoes and cook for 2 to 3 minutes, or until set and golden around the rim.
Transfer the skillet to the oven and bake the frittata for 15 minutes, or until it is just set in the center. Let cool slightly, then turn the frittata out onto a plate. Invert the frittata again onto another plate so that it is right side up. Repeat with the remaining olive oil, eggs and roasted potatoes to make a second frittata. Serve the frittatas in wedges with the Romesco Sauce and thickly sliced dry-cured chorizo
The potato frittatas can be loosely covered with aluminum foil and stored at room temperature for up to 2 hours before they are set out on the buffet table.