Photograph: Karen Thomas
Boil the peas in salted water for 3 minutes until just cooked. Drain and refresh in cold water. Set aside half of the peas.
Transfer the remaining peas to a food processor with the crème fraîche, parmesan, half the lemon zest, 1 tablespoon of the lemon juice and a good pinch of sea salt. Whiz together for about 3 minutes until smooth.
Tip the pea purée into a bowl and leave it to rest at room temperature for 20 minutes – the flavour will probably mellow, so add a little more grated parmesan, lemon juice and salt to taste. You want a strong pea flavour but a good amount of 'punch' to cut through the cheese.
Put the watercress and reserved peas in a bowl. Add the olive oil and some seasoning, and toss altogether.
To serve, tear up the mozzarella balls and arrange on a large serving platter. Drizzle with a little more olive oil and a little lemon juice, and season with sea salt and black pepper. Spoon some of the purée over the cheese. Scatter over the watercress and pea salad, then sprinkle with the remaining lemon zest and parmesan.Serve with griddled bread drizzled with extra-virgin olive oil.
Get ahead: make the purée a few hours ahead; chill. Bring back to room temperature before serving.