Spinach-watercress dip becomes a handheld salad when scooped up by lettuce leaves.
Wash spinach; drain, but do not dry. Transfer to a large stockpot. Cover, and cook over medium-high heat, stirring occasionally, until wilted, about 5 minutes. Transfer to a clean kitchen towel, and squeeze out excess liquid. Let cool slightly; coarsely chop. Set aside.
Heat oil in a medium saute pan over medium heat. Add onion, and cook, stirring occasionally, until soft and translucent, about 8 minutes. Transfer to a plate, and let cool slightly.
Whisk together yogurt and lemon zest and juice in a medium bowl. Stir in spinach, onion, and watercress; season with salt and pepper. Garnish with watercress sprigs, and serve with lettuce leaves for dipping.
Dip can be refrigerated up to 6 hours.