1. Whisk together the vinegar, olive oil, lemon juice, brown sugar, mustard, Worcestershire sauce, rosemary and minced garlic and half the black pepper in a small bowl. Place the steak in a resealable bag, pour in the marinade and turn to coat. Refrigerate for a minimum 4 hours, turning occasionally.
2. Heat a grill pan over medium high heat and mist with cooking spray. Remove steak from marinade and season with 1/8 teaspoon of the salt. Grill 5 minutes per side for medium rare or 7 minutes for medium. Transfer steak to a cutting board, sprinkle with another 1/8 teaspoon salt, tent with foil and let rest 10 minutes.
3. Melt the butter in a large nonstick skillet over medium heat. Add the sliced garlic and saute 1 minute; season with remaining salt and black pepper. Add half the spinach and saute until wilted, about 1 minute. Add remaining spinach and saute until just wilted, 1 minute more. Slice steak and serve with the spinach.