Ingredients

• ¹/₄ cup unsalted butter
  
• 1 medium (or 2 small) yellow onion, finely diced
  
• 12 ounces baby bella mushrooms, finely diced (stems included!)
  
• 2 tablespoons all-purpose flour
  
• ¹/₂ cup heavy cream
  
• Kosher salt and freshly ground black pepper
  
• 1 1 (17.3-oz / 490-gram) package puff pastry, thawed but still cold