To melt the cheese on top of the French Onion soup: -Place the cheese on top of the very hot soup and it will start to melt on its own. -If the soup is not hot enough to melt the cheese, you can place the soup in a microwave-safe bowl and microwave on high for 15-30 seconds until the cheese has melted. -If the soup is not hot enough to melt the cheese, you can place the soup in an oven-safe bowl on a baking sheet and place it in the oven on broil for about a minute or you can place the bowl of soup into the Instant Pot on the metal rack and use the crisping lid to broil the cheese until it's nice and melted.