In a large bowl, whisk together the olive oil, mustard and vinegar and season with black pepper.
In a separate bowl, mix the crème fraîche with half the lemon juice. Set aside. Divide the watercress between each plate and drizzle with the dressing.
Heat a dry frying pan until hot. Add the pancetta and fry for 2 minutes until it begins to release some fat, then add the scallops and fry for 30 seconds to 1 minute on each side, until opaque and just cooked through. Add the remaining lemon juice to the pan.
Place 3 scallops on each plate of watercress, spooning over the pancetta and pan juices. Serve with the lemon crème fraîche and garnish with the zest.