Roasted And Shredded Beef Sandwiches

Name: Roasted And Shredded Beef Sandwiches
Ingredients: 3 pounds beef chuck shoulder roast, cut into 1 pound pieces; 4 cups low sodium beef broth; 1 head garlic, cloves smashed and peeled; small handful of flat leaf Italian parsley and stems; 1 thick slice red onion; 3 fresh sage leaves; 1 1/2 tablespoons fresh thyme leaves, and a few fresh stems as well; coarse sea salt and fresh ground black pepper; 1/2 cup mayonnaise; 1-2 tablespoons prepared horseradish; 1 tablespoon chopped chives; 1 tablespoon chopped parsley; pinch of salt and fresh ground pepper to taste; brioche buns, red onion slices, and dijon mustard for serving