Bring the flank steak to room temperature.
Prepare an outdoor grill or preheat a stovetop grill pan.
Season both sides of the steak with salt and freshly ground pepper. Drizzle 1 tablespoon of the olive oil on 1 side of the meat and lightly rub it in.
Sprinkle half of the oregano on top and place the steak on the grill, herb side up.
Grill on 1 side for about 5 minutes or until the juices of the meat come to the surface of the steak. Turn the steak once and continue cooking for about 1 to 2 minutes, or until done to your taste. Let rest for a few minutes.
Meanwhile: In a large saute pan, heat 2 tablespoons olive oil until hot. Add the garlic and saute about 1 minute, or until light brown. Add the greens and toss occasionally.
After about 3 minutes, the greens will have cooked down. Season with a large pinch of salt and a sprinkle of freshly ground pepper. Cook another 3 minutes. Splash greens with 2 tablespoons red wine vinegar and hot sauce, to taste, if using. Transfer the greens to a plate.
For the crostini: Brush both sides of the bread slices with olive oil and season with salt and pepper. Place the slices on the grill or grill pan and cook until crispy, about 1 minute on each side.
Cut the flank steak across the grain in 1/4-inch slices. Place a slice of steak on top of each crostini, top each with the greens and finish with a drizzle of olive oil and hot sauce, as desired. Can be served warm or at room temperature.