Grilled Cheese and Short Rib Sandwiches with Pickled Caramelized Onions and Arugula

Name: Grilled Cheese and Short Rib Sandwiches with Pickled Caramelized Onions and Arugula
Ingredients: 5 pounds beef short ribs; 1/4 cup (1/2 stick) butter; 3 celery stalks, chopped; 2 large carrots, peeled, coarsely chopped; 1 medium onion, chopped; 1 1/4 cups dry red wine; 1/2 cup low-salt beef broth; 1/3 cup medium-dry Sherry; 2 garlic cloves, peeled; 2 bay leaves; 1 large fresh thyme sprig; 1 tablespoon butter; 2 large red onions, halved, thinly sliced crosswise (about 6 cups); 4 1/2 tablespoons red wine vinegar; 1 1/2 teaspoons sugar; Butter, room temperature; 16 slices country-style crusty white bread; 12 ounces Petit Basque or Monterey Jack cheese, sliced; 4 cups baby arugula