French Onion Tart with Gruyere (Flaky Puff Pastry) - Quickie Recipe

Ingredients

• 3 tablespoons unsalted butter
• 1 tablespoon olive oil
• 2 lb (900 g) yellow onions, thinly sliced
• 3/4 teaspoon kosher salt, plus more to taste
• 1 teaspoon fresh thyme leaves, plus more for garnish
• 1/2 teaspoon freshly ground black pepper
• 2 teaspoons sherry vinegar
• 1 sheet frozen puff pastry, about 14 oz (400 g), thawed but cold
• 1 tablespoon all-purpose flour, for rolling
• 1 tablespoon Dijon mustard
• 5 oz (140 g) Gruyere cheese, coarsely grated
• 2 tablespoons chopped fresh chives