Cook potato in a large saucepan of boiling water for 15 mins or until tender. Drain well. Return potato to the pan and place over low heat. Cook for 1 min. Remove from heat. Use a potato masher or fork to coarsely mash the potato. Add the milk and mash until smooth. Season. Cover to keep warm.
Heat butter in a large frying pan over medium-low heat until the butter begins to foam. Cook for 1 min. Add the garlic and cook for a further 1-2 mins or until the butter is nutty brown and the garlic is light golden. Transfer to a bowl.
Heat oil in the frying pan over high heat. Add the spring onion and cook, stirring, for 1 min or until the spring onion softens slightly. Add the rocket and cook, stirring constantly, for 1-2 mins or until rocket just wilts. Season. Stir through the lemon juice.
Heat a clean large frying pan over medium-high heat. Spray with olive oil spray. Season the salmon. Cook for 3 mins each side for medium or until cooked to your liking. Transfer to a plate and set aside for 5 mins to rest.
Add rocket mixture to the potato mixture in the pan and stir to combine. Transfer to a large serving plate. Drizzle with the burnt garlic butter. Serve immediately with the salmon.
