Rinse watercress and drain well. Pick tender sprigs from stems and cut into 3- to 4-inch lengths; you should have about 6 cups sprigs. Finely chop enough of the stems to make 2 tablespoons; save remainder for other uses or discard.
In a large bowl, mix olive oil, lemon juice, garlic, and chopped watercress stems. Add watercress sprigs and mix. Add salt and pepper to taste.