Place the cutlets in a large bowl, season and add the thyme, lemon zest and juice. Cover and marinate in the refrigerator for 30 minutes.
Meanwhile, prepare the potato salad; boil the potatoes in a large pan for 15-20 minutes. Drain, cool and cut into quarters.
Place the peas in a food processor or blender with the olive oil, lemon juice and mint or chives. Whisk together until roughly blended. Season. Combine the pea mixture with the potatoes and watercress or rocket leaves.
Heat a large non-stick griddle pan over a medium heat and cook the cutlets in batches for 6-8 minutes on each side
Divide the cutlets equally and serve with salad.
If preferred cook the cutlets under a preheated grill or on a prepared barbecue.