Cheesy French Onion Baked Potatoes

Name: Cheesy French Onion Baked Potatoes
Ingredients: 4 medium russet potatoes (8 to 10 ounces each); 1 pound yellow onions (about 1 large or 2 medium); 1/2 small bunch fresh thyme (about 10 sprigs), plus more for serving; 1 clove garlic; 2 ounces Gruyère cheese; 4 tablespoons unsalted butter, divided; 1/2 teaspoon kosher salt, plus more for the potatoes; 1/2 teaspoon freshly ground black pepper, plus more for the potatoes; 1 tablespoon all-purpose flour; 1 cup beef broth; 1/2 teaspoon balsamic or sherry vinegar