Preheat a non-stick pan or skillet over medium heat. Add a small amount of oil to prevent sticking. Once hot, add the chopped onion and cook until translucent, stirring occasionally. This should take around 5 minutes, during which the onion will start to soften and slightly brown (colour: light golden brown, smell: sweet and caramelised).

Once the onion is cooked, add the mashed avocado to the pan and stir to combine. Cook for a further 2-3 minutes, allowing the avocado to heat through and the mixture to thicken (texture: creamy and smooth, smell: nutty and earthy).

Add the flaked tuna to the pan, breaking it up with a spoon as you go. Stir to combine with the avocado mixture and cook for a further 2-3 minutes, allowing the tuna to heat through and the mixture to thicken (texture: chunky and slightly sticky, smell: fishy and slightly sweet).

Stir in the mayonnaise, chopped fresh dill, and chopped fresh chives. Season with salt and black pepper to taste. Cook for a further minute, allowing the flavours to combine and the mixture to thicken (colour: creamy white, smell: tangy and herby).

Meanwhile, toast the bread slices until lightly browned (colour: golden brown, smell: toasted and crispy). Assemble the sandwich by spreading the tuna-avocado mixture onto the toasted bread slices.

Top the sandwich with chopped iceberg lettuce and sliced tomato, if desired. Serve immediately and enjoy (colour: vibrant green and red, smell: fresh and crunchy).

For an extra layer of flavour, grill the sandwich in a pan or under the broiler until lightly toasted and crispy (colour: dark golden brown, smell: toasted and nutty).