Preheat 3 tablespoons of olive oil in a large skillet at medium heat. After a few minutes, add the potatoes. Season lightly with salt and pepper and continue cooking until browned on most spots.
Move the potatoes to one side of skillet. Add the chorizo on the other side and cook for 5 to 6 minutes, breaking into crumbles as it cooks. Add the onion, serrano and tomato to the chorizo side and cook for another 2 minutes.
Combine the chorizo with potatoes. In a bowl beat the eggs with a little splash of water and a pinch of salt and pepper. Stir into the chorizo/potato mixture until the eggs are cooked. Garnish with cilantro. Yields 6 servings.