Remove and discard the tough skin from the pancetta then cut the meat and its fat into short finger thick strips.
Warm the olive oil in a shallow non stick pan, add the pancetta and fry over a moderate heat for 4-5 minutes until the fat is golden.
Slice the spring onions into 1 cm pieces and add to the pancetta with the chopped thyme. Leave to cook for a minute or so.
Divide the watercress, salad leaves and sprouts between two plates.
When the pancetta is golden and sizzling tip in the cream and grind in a little black pepper. Add the grain mustard or vinegar if using. Bubble up for a minute or so, stirring.
Spoon the pancetta and sauce over the salad leaves and serve.