How to Make It
In a 10-inch nonstick skillet, cook the bacon in the olive oil over moderate heat, stirring occasionally, until golden brown, about 10 minutes. With a slotted spoon, transfer the bacon to a medium bowl.
Add the potatoes to the skillet in a single layer and cook over high heat until browned on the bottom. Reduce the heat to moderately high and cook, stirring occasionally, until tender, about 12 minutes. With a slotted spoon, add the potatoes to the bowl with the bacon.
Add the onion to the skillet and cook over moderate heat, stirring occasionally, until softened, about 5 minutes. Reduce the heat to low and stir in the bacon and potatoes. Spread in an even layer, then pour in the beaten eggs. Cook, without stirring, until the eggs are just set, about 6 minutes; they will be a little moist on top.
Run a rubber spatula around the edge of the eggs. Invert a 12-inch plate over the pan. Wearing oven mitts, quickly and carefully invert the skillet and plate to release the eggs. Slide the eggs back into the skillet and cook over low heat until set, about 4 minutes. Carefully slide the tortilla onto a serving plate.
In a large bowl, toss the watercress with 1 tablespoon of the vinegar and season with salt and pepper; add more vinegar to taste. Cut the tortilla into wedges, top with the watercress salad and serve.