Makes 12 servings
1 cup olive oil
1 medium yellow onion, chopped
2 pounds boiling potatoes (red skins or Yukon gold work well)
Salt and pepper to taste
1. Slice potatoes in half crosswise, and then into quarter-inch half-moon shapes.
2. Pour olive oil into a 12-inch cast-iron skillet, preferably with slanted sides, and heat over medium-high heat for two minutes. Add onion and cook for about 5 minutes. Turn heat down to medium; add potatoes and fry until just fork-tender, about 20 minutes.
3. Drain oil and reserve for later. Dump potatoes and onions into a bowl lined with a brown paper bag to drain and cool slightly.
4. Whip eggs in a large bowl. Season generously with salt and fresh-ground pepper (about 1/2 teaspoon of each). Add potato and onion mixture and stir gently. Let sit for 5 minutes.
5. Wipe out pan and add two tablespoons of the reserved oil. Heat on high, add egg mixture, and cook for 30 seconds, shaking the pan quickly to distribute potatoes evenly. Turn heat down to low and cook until barely set, about 15 minutes.
6. Finish the omelet under the broiler until the top is golden, about 5 minutes. Remove from heat and let cool for 10 minutes before flipping onto a plate. Let cool for 5-10 minutes before slicing into bite-size diamond shapes. Serve with toothpicks and spicy tomato sauce.