French Onion Soup & Bone Broth

Name: French Onion Soup & Bone Broth
Prep time: 25 min
Cook time: 3 h 10 min
Total time: 3 h 35 min
Cuisine: French, Mediterranean, European
Ingredients: 1 pound bones; 1 yellow onion, quartered; 2 carrots, washed and cut into thirds; 2 celery stalks, washed and cut into thirds; 2 tablespoons apple cider vinegar; 1/2 bunch parsley; 4 cloves garlic; 2 teaspoons black pepper; 6 cups water; 1 tablespoon kosher salt; 4 tablespoons unsalted butter; 3 large onions, thinly sliced; 1 teaspoon kosher salt; 2 tablespoons soy sauce; 1 teaspoon kosher salt; 1 teaspoon black pepper; 4 slices French bread; 4 slices provolone cheese; 1 cup Swiss cheese, grated