French Onion Cabbage Soup

Name: French Onion Cabbage Soup
Ingredients: 6 tablespoons vegetable oil , divided; 1 small head green cabbage, cored and thinly sliced (about 16 cups ), divided; 1 large sweet onion , thinly sliced (about 3 cups), divided; 3 tablespoons cider vinegar , divided; 7½ cups unsalted vegetable broth; 1 teaspoon reduced-sodium soy sauce; 1 teaspoon chopped fresh thyme , plus more for garnish; 1 teaspoon ground pepper , plus more for garnish; ¼ teaspoon salt; 5 cups cubed whole-wheat baguette (½-inch); ¾ cup shredded Gruyère cheese