Prepare the Tuna Filling: In a medium mixing bowl, add the thoroughly drained tuna. Use a fork to flake it apart. Add the mayonnaise, chopped red onion, chopped gherkins, Dijon mustard, lemon juice, and fresh parsley.

Mix and Season: Stir everything together until well combined. Season with a small pinch of salt and a generous amount of freshly ground black pepper. Taste the mixture and adjust the seasoning if needed. It should be creamy, tangy, and savoury.

Butter the Bread: Lay the four slices of bread on a chopping board. Spread the softened butter evenly over one side of each slice, going right to the very edges. This will be the outside of your sandwich.

Assemble the Sandwiches: Take a large, non-stick frying pan and place it over a low heat. Place two slices of bread in the pan, butter-side down. I find that starting with a cold pan and heating it gently helps the bread toast more evenly.

Layer the Ingredients: Working quickly, sprinkle half of the grated cheddar over each slice of bread in the pan. Divide the tuna mixture evenly between the two slices, spreading it over the cheese. Top the tuna mixture with the remaining grated cheese.

Grill the First Side: Place the remaining two slices of bread on top, butter-side up. Increase the heat to medium-low. Cook for 4-5 minutes, or until the bottom is a deep golden brown and the cheese has started to melt. You can gently press down on the sandwich with a spatula to help it cook evenly.

Flip and Finish: Carefully flip the sandwiches over using a wide spatula. Cook for another 3-4 minutes on the second side until it's equally golden and crisp, and the cheese is completely melted and bubbling at the edges.

Rest and Serve: Remove the sandwiches from the pan and let them rest for a minute. This allows the filling to set slightly, making it less messy to cut. Slice in half diagonally and serve immediately while hot and gooey.