Prepare the turkey rashers: Lay the turkey bacon in a single layer in a cold, large frying pan. Turn the heat to medium and let them cook slowly. You'll hear a gentle sizzle as the fat renders, and the edges will start to curl and turn a deep golden brown. Cook for 4–5 minutes per side, until the rashers are crisp and the kitchen smells wonderfully smoky. Transfer to a plate lined with kitchen paper.

Toast the bread: Wipe the pan clean with kitchen paper, then return it to a medium heat. Spread one side of each bread slice with the softened butter. Place the bread, buttered side down, into the hot pan. Listen for a soft, even hiss. Toast for 2–3 minutes until the underside is a rich, even golden brown with darker patches at the edges. Flip and toast the other side for a further minute. The bread should feel firm and sound hollow when tapped.

Prepare the tomatoes and lettuce: While the bread toasts, slice the tomatoes into 1cm-thick rounds. You should get about 3 slices per tomato. Lay them on a plate and sprinkle with a pinch of sea salt flakes. Wash the lettuce leaves and pat them completely dry with a clean tea towel—any moisture will make the bread soggy.

Make the spread: In a small bowl, combine the mayonnaise with the Dijon mustard if using. Stir until smooth and pale. This simple step adds a subtle tang that cuts through the richness of the turkey rashers. Set aside.

Assemble the sandwich: Lay two slices of toasted bread on a clean board, toasted side up. Spread a generous layer of the mayonnaise mixture onto each slice. Layer on the lettuce leaves, pressing them down gently so they stay flat. Arrange the tomato slices on top of the lettuce, slightly overlapping if needed. Grind a little black pepper over the tomatoes.

Add the turkey rashers and finish: Place four crisp turkey rashers on each sandwich, breaking them in half if they are very long so they lie flat. Top with the remaining two slices of bread, toasted side down. Press down firmly but gently with the palm of your hand to settle everything together.

Slice and serve: Using a sharp serrated knife, cut each sandwich diagonally—this creates two neat halves with appealingly exposed layers. The sound of the knife cutting through the crisp toast and the soft vegetables is deeply satisfying. Serve immediately, while the bread is still warm and the rashers are at their crispiest.