Peel and chop the potato into bite-sized pieces. Rinse in water and drain. Place in a microwave-safe container with 2 tbsp water and cover loosely with plastic wrap. Microwave at 600W for 5 minutes, then drain.

Return the potato to the container and mash while hot, adding butter and milk. Mix until smooth, then add salt and pepper. Let cool to room temperature.

Cut the avocado into 1cm cubes and mash in a bowl with lemon juice, mayonnaise, salt, and pepper until smooth.

Finely chop the onion and rinse in water. Drain well.

Cut the salmon into 1cm cubes and mix in a bowl with the onion, olive oil, and salt.

Prepare the lemon slices by cutting halfway through each slice and twisting them into decorative shapes.