Roast the tomatoes with olive oil and rosemary until tender to add an extra dose of juicy flavor.
Heat oven to 400° F. Toss the tomatoes, 1 tablespoon of the oil, rosemary, and ¼ teaspoon salt in a 9-by-13-inch baking dish. Roast until very tender, 20 to 25 minutes. Let cool.
Meanwhile, combine the sour cream, horseradish, and ¼ teaspoon pepper in a small bowl.
Toss the arugula, lemon juice, and the remaining tablespoon of oil in a medium bowl.
Divide the tomatoes, horseradish cream, arugula, and roast beef among the bread.