Ingredients

• 1 bunch scallions
• 2 tablespoons extra-virgin olive oil
• 2 medium onions, chopped (1 1/2 cups)
• 4 garlic cloves, finely chopped
• 1 1/4 teaspoons salt
• 1/2 teaspoon black pepper
• 6 oz Spanish chorizo (cured spiced pork sausage), cut into 1/4-inch dice
• 1 1/2 lb yellow-fleshed potatoes such as Yukon Gold (about 4 medium), peeled and cut into 1/4-inch dice
• 9 large eggs
• 3/4 cup sour cream
• 5 oz Manchego or white Cheddar, coarsely grated (2 cups)

Instructions

1. Put oven rack in middle position and preheat oven to 400°F. Lightly oil a 13- by 9-inch baking pan or dish.
2. Finely chop white and green parts of scallions and reserve separately.
3. Heat oil in a 12-inch heavy skillet over moderately high heat until hot but not smoking, then cook onions, garlic, white parts of scallions, 3/4 teaspoon salt, and 1/4 teaspoon pepper, stirring occasionally, until onions are golden, about 6 minutes. Add chorizo and cook, stirring occasionally, until just beginning to brown and release oil, about 4 minutes. Reduce heat to moderate, then stir in potatoes and cover skillet. Cook, stirring occasionally, until potatoes are tender, about 10 minutes. Transfer mixture to a large bowl and cool slightly.
4. Whisk together eggs, sour cream, cheese, scallion greens, and remaining 1/2 teaspoon salt and 1/4 teaspoon pepper in a bowl, then pour into baking pan. Sprinkle potato mixture over eggs (some potatoes will stick out). Bake until custard is set, about 20 minutes. Cool in pan on a rack to warm or room temperature. Trim 1/2 inch off each side, then cut tortilla into 1 1/2- by 1-inch rectangles.