Flatten the steaks slightly with a meat mallet. Season both sides with salt and pepper.
Preheat a barbecue plate over medium-high heat. Combine oil and garlic in a small bowl. Brush mushrooms and steaks with the garlic oil.
Barbecue steaks for 2 minutes on each side (for medium-rare) or until cooked to your liking. Transfer to a plate, cover with foil and rest for 5 minutes. Barbecue mushrooms for 2-3 minutes on each side until tender. Transfer to a plate.
Spray both sides of the bread with oil and barbecue 1-2 minutes each side or until golden. Cut steaks in half. Spread 4 slices of toasted bread with hummus. Top with watercress, steaks and mushrooms. Season with salt and pepper. Cover with remaining bread and serve.