French Onion Tart

Name: French Onion Tart
Ingredients: 1½ lb. onions (about 4 small), preferably yellow, halved, thinly sliced; 3 Tbsp. unsalted butter; ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt, plus more; 1 garlic clove, finely chopped; 2 tsp. sherry vinegar or red wine vinegar; 2 tsp. Worcestershire sauce; 1 tsp. freshly ground pepper; 4 oz. Gruyère, coarsely grated, divided; 1 sheet frozen puff pastry (one 14-oz. package or half of a 17.3-oz. package), thawed; 1 Tbsp. Dijon mustard; Chopped chives (for serving)