How to Make It
Heat a grill pan over medium-high heat; coat with cooking spray. Sprinkle steak with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add steak to pan; cook for 3 minutes on each side. Let stand 5 minutes. Cut steak across grain into thin slices.
Place butter in a bowl. Microwave at HIGH 10 seconds. Add 1/8 teaspoon salt, 1/8 teaspoon pepper, 1/2 teaspoon rind, juice, and anchovy paste; stir until combined. Set aside.
Heat a skillet over medium-high heat. Add oil. Add onion; sauté 2 minutes. Add garlic and red pepper; sauté 30 seconds. Stir in broth and rabe; bring to a boil. Cover and simmer 3 minutes. Remove from heat; stir in remaining rind and salt.
Place 3 ounces steak and 1/2 cup rabe on each of 4 plates. Top each steak with 1 1/2 teaspoons butter.