Prepare the Cheeses: In a small bowl, combine the grated mature Cheddar and grated Gruyère. Toss them together with a pinch of black pepper until evenly mixed. This ensures you get a perfect blend of flavour in every single bite.

Make the Garlic Butter: In another small bowl, mix the softened unsalted butter with the garlic powder and a pinch of salt until well combined.

Assemble the Sandwich: Lay your two slices of bread on a chopping board. Spread the garlic butter evenly over one side of each slice – these will be the inside of your sandwich.

Add the Filling: Pile the mixed cheese onto the buttered side of one slice of bread, spreading it out to the edges. Place the second slice of bread on top, buttered-side down.

Prepare the Exterior: This is the crucial step. Using a spatula or the back of a spoon, spread the mayonnaise in a thin, even layer over the entire outer surface of both the top and bottom slices of the sandwich. I know it sounds unusual, but trust me on this!

Cook the First Side: Place a non-stick frying pan over a low-medium heat. What works best for me is using a heavy-bottomed skillet for even heat. Carefully place the sandwich in the dry pan. Cook for 4-5 minutes, or until the bottom is a deep golden brown and beautifully crisp. Don't be tempted to press down on it too much.

Flip and Finish: Carefully flip the sandwich over using a wide spatula. Cook for another 3-4 minutes on the second side, until it's equally golden and the cheese inside is completely melted and gooey. You can cover the pan with a lid for a minute to help trap the heat and encourage the cheese to melt fully.

Rest and Serve: Slide the grilled cheese onto your chopping board and let it rest for a minute. This helps the molten cheese to set just slightly, preventing it from spilling out everywhere. Slice in half diagonally and serve immediately.