Light a grill or preheat a broiler. In a small bowl, combine 1 tablespoon of the olive oil with the mustard, garlic and crushed red pepper. Season with salt and black pepper. Put the steak in a shallow baking dish and spread the mustard mixture all over it. Grill the steak over high heat, turning once, until charred and medium rare, about 4 minutes per side. Transfer the meat to a cutting board and let rest.
Meanwhile, spread the tomato slices on a large platter, drizzle with the balsamic vinegar and the remaining 1 tablespoon of olive oil and season with salt and black pepper. In a small bowl, mash the Gorgonzola with the mayonnaise. Spread the Gorgonzola mixture on the cut sides of the baguette lengths and grill, cheese side up, until the baguette is toasted and the Gorgonzola is melted, about 1 minute, being careful not to burn the bread.
Cut the steak into 4-inch lengths, then thinly slice it across the grain. Pile the meat on the baguettes and spoon the accumulated meat juices over the top. Top with the tomato slices and watercress, close the sandwiches and serve.