Caramelized Onion and Gruyère Tartlets - Great Easy Recipe

Ingredients

• 2 large onions (thinly sliced)
• 2 tbsp unsalted butter
• 1 tbsp olive oil
• 1 tsp sugar (optional, to enhance caramelization)
• ½ tsp salt
• ¼ tsp black pepper
• 1 tsp balsamic vinegar (for extra depth, optional)
• 1 sheet puff pastry (thawed)
• 1 cup Gruyère cheese (grated)
• 1 egg (beaten, for egg wash)
• 1 tsp Dijon mustard (optional, for extra tang)
• ½ tsp fresh thyme leaves (for garnish)