Heat grill to medium-high. In a large bowl, toss the mushrooms, green beans, onion, 1 tablespoon oil, ½ teaspoon salt, and ¼ teaspoon pepper.
Grill the vegetables (place the beans perpendicular to the grates to keep them from falling through), turning occasionally, removing them as they become tender, 8 to 12 minutes.
Meanwhile, season the steak with ½ teaspoon salt and ¼ teaspoon pepper. Grill alongside the vegetables, brushing occasionally with 2 tablespoons vinegar, 3 to 5 minutes per side for medium-rare. Let rest for 5 minutes before slicing.
Divide the watercress, grilled vegetables, and steak among bowls. Drizzle with the remaining 2 tablespoons oil and 2 tablespoons vinegar.