Toast the bread slices on a medium barbecue for 1 minute on each side until browned. Set aside, keeping warm.
Place the mushrooms on the barbecue, gills down, and cook for 10 minutes. Turn the mushrooms over and cook for another 10 minutes. (The undersides are cooked first to seal in all the juices.)
Place the toasted bread on 4 plates and spread each slice with some goats' cheese. Mix together the olive oil, vinegar and seasoning in a small jug to make a dressing.
Remove the cooked mushrooms from the barbecue and slice. Mix them with the watercress and dressing and heap on to the toasted bread. Serve immediately.