Heat grill to high. Rub meat with oil and season with salt and the black pepper. Grill on both sides until slightly charred and cooked to medium-rare doneness, about 4 minutes per side. Remove from the grill and let rest 5 minutes. Slice into 1/4-inch thick slices.
Lay toasted croissant halves on a flat surface and top each half with a slice of the cheese. Divide the steak among 4 of the slices and combine halves make 4 sandwiches. Wrap each sandwich in foil, place on the grill and grill for 1 minute on each side. Remove from the grill and foil, lift 1 slice of the bread from each sandwich, and spread with a few tablespoons of Horseradish Mayonnaise. Top with a few sprigs of watercress and serve.
Whisk together all ingredients in a small bowl. Season, to taste, with salt. Cover and refrigerate for at least 30 minutes before using.