Mushroom and Cheesy Onion Pastry – Sana

Ingredients

• 4 tablespoons salted butter, divided
• 2 tablespoons vegetable oil, divided
• 5-6 ounces crimini or button mushrooms, sliced
• 2 medium yellow onions, thinly sliced
• ¼ teaspoon dried thyme
• 1 tablespoon sugar
• 1½ tablespoon balsamic vinegar
• ¼ teaspoon garlic powder
• salt and pepper, to taste
• mozarella cheese (as per your liking)
• 1 package (1 pound) frozen puff pastry, thawed
• 1 egg, lightly beaten