In an 8- or 9-in. glass dish, mix olive oil, lemon juice, garam masala, 1/2 tsp. salt, and the pepper. Turn steak in mixture and let marinate at room temperature 1 hour.
Prepare a charcoal or gas grill for very high heat (550° to 650°; you can hold your hand 5 in. above cooking grate only 1 to 2 seconds). While grill heats, preheat cooking grate for at least 10 minutes.
Pour marinade into a small pan and bring to a simmer; remove from heat. Pour 1 tbsp. water into a blender, turn on, drizzle in marinade, and whirl to emulsify.
Grill steak, covered, turning once, about 5 minutes for medium-rare (cut to test). Lift to a board and loosely cover with foil.
Grill naan, turning once, until hot and slightly crusty, 1 to 2 minutes. Lift to board.
Thinly slice steak across the grain. Let each person arrange meat and watercress over half of each naan. Drizzle sauce on top, sprinkle with salt to taste, and fold naan in half to make a sandwich.
Note: Nutritional analysis is per naan-wich.