Spray the griddle pan with low calorie cooking spray and place over a medium heat. Place the bacon medallions in the pan and cook for 1 - 2 minutes on each side or until cooked.
Unfold the flatbread and place a bacon medallion on one half of each.
Divide the cheese, tomato and spring onion between each and place on top of the bacon. Spread the mustard over the other half of the flatbread and fold over the filling.
Place on the preheated griddle pan, pressing down to produce griddle marks.
Cook for 3 - 5 minutes, turning occasionally, until cheese has melted. Remove from pan and carefully lift the top and add some watercress. Serve immediately.