Heat a grill or grill pan over medium-high heat. Brush the steak with 1 tablespoon of the olive oil and season with salt and pepper. Grill for 5 minutes on each side for medium-rare; set aside.
In a medium bowl, whisk together the Worcestershire sauce, 1/2 teaspoon of salt, the garlic, the mustard and the remaining 3 tablespoons of olive oil. Add the steak and marinate for 15 minutes.
In a large bowl, combine the cucumber, bell pepper and corn.
When the steak is done marinating, place it on a cutting board and pour the marinade into a small saucepan set over high heat to simmer, about 1 minute. Slice the steak into 1/4-inch-thick strips and add to the vegetables. Add the marinade, mint and salt and pepper to taste, and toss.
Divide the greens among 4 dinner plates. Drizzle with olive oil and salt, then top with the steak strips and vegetables.