French Onion French Bread Pizza

Name: French Onion French Bread Pizza
Ingredients: 8 tbsp. (1 stick) salted butter; 3 lb. onions, thinly sliced; 1 tsp. kosher salt; 1/2 tsp. black pepper; 2 tbsp. balsamic vinegar; 2 tsp. chopped fresh thyme, plus more for topping; 1 c. beef broth; 2 (15-inch) loaves soft French bread, halved lengthwise and then crosswise (8 pieces total); 1/2 tsp. garlic powder; 1/2 c. shredded gruyère cheese (about 2 ounces); 1/2 c. shredded mozzarella cheese (about 2 ounces); 1/4 c. pitted kalamata or Niçoise olives, sliced into rings