Preheat a panini press or griddle. Bring a large pot of water to a boil. Add the broccoli rabe and cook until bright green, about 3 minutes. Drain well.
In a large skillet, heat the 1/4 cup of oil. Add the minced garlic, anchovy paste and crushed red pepper (if using) and cook over moderately high heat for 10 seconds. Add the broccoli rabe and season with salt and pepper (if desired). Cover and cook over moderately high heat, stirring occasionally, until the broccoli rabe is tender, about 5 minutes.
Layer 2 slices of provolone on the bottom of each slice of bread. Slather on some dijon mustard. Top with the broccoli rabe, beef, bell peppers and the remaining provolone. Close the sandwich and brush lightly with oil. Grill the sandwiches until toasted and the cheese is melted, about 7 minutes. Serve hot.