Bring a large saucepan of salted water to the boil over high heat.
Add peas and cook for 3 minutes.
Drain peas and rinse with cold water.
Place half the peas in a food processor with crème fraiche, 3 tablespoons parmesan, the zest of one lemon, 2 tablespoons lemon juice and a pinch of salt. Blend until smooth.
Put pea puree in a bowl and set aside.
Add remaining peas and watercress to a large bowl. Add 4 teaspoons olive oil and season with salt and pepper. Toss to combine.
Tear mozzarella balls and place on a large plate or serving platter. Drizzle with the remaining olive oil and some lemon juice. Spoon puree over the cheese and then scatter over watercress and peas. Sprinkle with remaining lemon zest and parmesan.