Borscht with Roast Beef Toasts

Name: Borscht with Roast Beef Toasts
Ingredients: 2 tablespoons extra-virgin olive oil, plus more for the bread; 1 bunch scallions, chopped; 1 large russet potato, peeled and cut into 1/2-inch cubes; 1 14.5-ounce can low-sodium beef broth; 12 ounces packaged cooked and peeled beets, cut into 1/2-inch cubes, juices reserved; Kosher salt and freshly ground pepper; 1/3 cup sour cream; 1 tablespoon drained horseradish; 4 slices marble rye bread; 1 tablespoon red wine vinegar; 1 tablespoon chopped fresh dill, plus more for topping; 8 ounces deli-sliced roast beef