How to Make It
Heat a grill pan over medium-high. Combine salt, pepper, paprika, and garlic powder in a small bowl. Rub spice mixture evenly over steak. Add steak to pan; grill 5 minutes on each side for medium-rare or until desired degree of doneness. Place steak on a cutting board. Let stand 5 minutes. Cut across the grain into thin slices.
Combine oil, vinegar, and mustard in a large bowl, stirring with a whisk. Add steak, arugula, and onion; toss to coat. Divide salad among 4 plates. Top evenly with avocado.