Despite its refined appearance, this crisp galette is a lot like a big potato pancake -- and just as irresistible. It gets its oniony depth from the leek.
Place potato in a bowl of cold water, and let soak for 10 minutes. Drain well in a salad spinner or squeeze in a clean kitchen towel to remove excess water. Combine potato, leek, flour, nutmeg, 3/4 teaspoon salt, and 1/4 teaspoon pepper.
Heat 2 tablespoons oil in a large nonstick skillet over medium heat. Scatter potato mixture in skillet, and press lightly with a spatula to make sure it holds together. Cook until underside is golden, about 6 minutes. Flip. Raise heat to medium-high. Cook until underside is golden, 4 to 5 minutes.
Turn out galette. Toss watercress with lemon juice and 1 1/2teaspoons oil, and place on top of galette. Slice into 8 wedges.