Sprinkle roast with salt and pepper; in large skillet, heat oil over medium-high heat. Add roast to skillet and cook 10 minutes or until outside is seared, turning to sear all sides.
Place onion in 5-6 quart slow cooker; top with roast, rosemary and broth. Cover and cook on high 5-6 hours or on low 10-12 hours or until beef is tender.
Preheat oven to 350°; spread cut sides of bread with butter and place cut side up on rimmed baking pan. Bake bread 6 minutes or until toasted.
Remove and discard rosemary; transfer roast to cutting board and use 2 forks to pull apart. Strain onion and juices in slow cooker through fine-mesh strainer into bowl, reserving onion and juices.
Evenly top bottom halves of bread with horseradish sauce, beef and onion; spoon about 2 tablespoons strained juices over onion on each sandwich. Serve sandwiches with small bowls of remaining juices for dipping.
Approximate nutritional values per serving: 897 Calories, 33g Fat (11g Saturated), 158mg Cholesterol, 1770mg Sodium, 82g Carbohydrates, 1g Fiber, 69g Protein