1. Combine parsley, cilantro, 2 tablespoons oil, lemon rind, 1 tablespoon lemon juice, 1/4 teaspoon salt, 1/4 teaspoon pepper, and 4 garlic cloves in a mini food processor; process until finely chopped.
2. Heat a grill pan over medium-high heat. Coat pan with cooking spray. Sprinkle steak with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add steak to pan; cook 5 minutes on each side or until desired degree of doneness. Let steak stand 5 minutes. Thinly slice steak across the grain.
3. Mince remaining 1 garlic clove. Combine minced garlic, remaining 1 tablespoon extra-virgin olive oil, remaining 2 teaspoons lemon juice, Dijon mustard, remaining 1/4 teaspoon freshly ground black pepper, and remaining 1/8 teaspoon salt in a medium bowl; stir with a whisk. Add baby spinach and sliced red onion to bowl; toss gently to coat. Serve steak with salad and pesto.