Chef John's Beef on Weck Sandwiches

Ingredients (9)

• + 2 pounds beef top-sirloin roast add to pantry
• + salt and pepper to taste add to pantry
• ✓ salt and pepper to taste
• + 1 tablespoon vegetable oil add to pantry
• + 2 teaspoons all-purpose flour, or as needed add to pantry
• + 2 ½ cups cold beef broth add to pantry
• + 2 teaspoons aged balsamic vinegar, or to taste add to pantry
• + 4 kummelweck dinner rolls, split add to pantry
• + ¼ cup extra-hot prepared horseradish, or to taste add to pantry