Combine the remaining 1 teaspoon oil, remaining 2 teaspoons marjoram (or oregano), remaining 1/2 teaspoon lemon zest, remaining 1/4 teaspoon salt and pepper, rosemary and minced garlic in a small bowl.
Rub on both sides of steak.
Rub both sides of bread with the halved garlic clove; discard the garlic.
Grill the steak 3 to 5 minutes per side for medium-rare.
Grill the bread until toasted, 30 seconds to 1 minute per side.
Transfer the toasts to a large plate, place the steaks on top of the toasts and spread the herb butter on top of the steaks; let rest for 5 minutes.
Divide watercress among 4 plates and top with the steak-topped toasts.