Preheat a grill pan or an outdoor grill to medium-high heat and preheat oven to 400F.
Season the steak on both sides with salt, freshly ground pepper and grill seasoning. Drizzle with EVOO and brush to coat in the oil. Transfer the steak to the grill and cook on each side for about 5-6 minutes for medium rare. Remove the steak from the grill, cover loosely with aluminum foil and let it rest.
While the steak is grilling, heat a medium skillet over medium-heat with one turn of the pan of EVOO, about 1 tablespoon. Add the onions and season with salt and pepper. Cook for about 7-8 minutes, stirring every now and then. Add peppers and continue to cook until veggies are soft and onions are caramel in color, about 10 minutes. Add the beef stock, turn up the heat to high and continue to cook until there is only about 1/4 cup of liquid left in the skillet.
Once you have the onions and peppers going, make the crostini: Drizzle the bread slices with some EVOO and place on the grill until you have light grill marks, about 1 minute on each side. Remove from the grill and reserve on a cookie sheet.
To assemble the crostini, slice the meat against the grain on an angle with a sharp knife into very thin slices. Fold each slice of meat in half and place on top of the grilled bread slices. Top with the onion and pepper mixture and some shredded provolone cheese.
When you are ready to serve, place the crostini under the broiler or in a 400F oven to melt the cheese. Serve immediately.