In a 10 to 12 inch nonstick skillet, heat 1 tsp. oil and saut the onions over medium heat until translucent, about 3 minutes. Do not brown.
Lower the heat and stir in the watercress. Cover and cook for 3 additional minutes. Take mixture off the heat and let cool slightly. Clean pan.
Meantime, in a large bowl, beat the egg whites, eggs, dill, and pepper until blended. Stir in the feta cheese and the watercress mixture.
Coat the bottom of the skillet with the remaining teaspoon of oil and return it to medium heat. When the skillet is hot, pour in the watercress egg mixture. Cover and cook for 5 to 8 minutes until the edges are firm and the bottom is golden and beginning to brown.
Place a large, flat plate or pizza pan over the skillet and flip the skillet over so that the frittata falls onto the plate. Slide the frittata back into the skillet and cook for about 5 minutes, until the eggs are fully cooked. Serve immediately or at room temperature.