Combine first 5 ingredients in a large, heavy-duty zip-top plastic bag. Add steak and seal bag, turning to coat. Refrigerate 6 hours or overnight.
Preheat grill to high (450° to 600°). Remove steak from marinade, discarding marinade. Pat steak dry with paper towels, and sprinkle with salt and pepper. Grill, on greased grill rack, 6 minutes on each side (for medium-rare) or to desired degree of doneness. Let stand 5 minutes before cutting into thin slices.
Toss spinach with 1/4 cup Dijon-Rosemary Dressing, and divide among 4 salad plates. Place steak slices on plates, and top with crumbled blue cheese. Serve with grilled baguette slices and remaining dressing.