How to Make It
Preheat the oven to 350°. In a medium, nonstick ovenproof skillet, heat 3 tablespoons of the olive oil. Add the potatoes and cook over moderate heat, turning and stirring occasionally, until the slices are tender and beginning to brown, about 7 minutes. Add the onion and cook, stirring occasionally, until softened, about 3 minutes. Stir in the slices of serrano ham and season with salt and pepper.
Season the eggs with salt and pepper and gently stir them into the skillet. Cook over moderate heat until the eggs are just beginning to set at the edge, about 3 minutes. Transfer the skillet to the oven and bake the frittata for about 12 minutes, until the eggs are completely set.
Meanwhile, in a large bowl, whisk the vinegar with the remaining 2 tablespoons of olive oil. Add the watercress, season with salt and pepper and toss.
Cut the frittata into wedges and serve with the watercress salad on the side.