1. Preheat oven to 375° and grill to high.
2. Toss first 4 ingredients (through rosemary sprigs); place on a baking sheet. Roast the potatoes for 25 minutes or until golden; set aside.
3. Combine vinegar and next 6 ingredients (through sugar) in a blender. Add 1/2 cup oil in a thin stream; blend. Add 1/8 teaspoon each salt and pepper; set aside.
4. While potatoes roast, heat remaining 1 tablespoon oil in a pan over medium heat. Sauté onion 10-12 minutes or until caramelized; set aside.
5. Season steaks with remaining salt and pepper. Reduce grill to medium-high; cook steaks for about 7 minutes per side or until desired degree of doneness. Remove from grill; let rest 3 minutes before slicing into 1-inch cubes.
6. Toss lettuce with tomato and blue cheese; add the potatoes and onion. Drizzle with dressing; toss. Top with steak; serve.