Whisk the horseradish sauce.

Cut the bun in half lenghtwise.

Drizzle the salmon with lemon juice.

Top with arugula.

Distribute some horseradish sauce over arugula.

Bring a saucepan of water to boil and add some white wine vinegar.

Separate the yolk from the white and place it in a small cup. Carefully slide it into the hot water.
Poach the yolk, occasionally turnig it with a spoon, for about 2-3 minutes.

Place the poached yolk on your sandwich and sprinkle with chopped spring onion.