Place potato in a large saucepan and cover with water. Bring to boil on medium heat. Cook for 15 minutes, until tender. Drain and shake over low heat until dry. Mash well and beat in milk. Add 1 teaspoon of wasabi paste and season to taste. Cover and keep warm.
Meanwhile, pat salmon dry with paper towel. Rub with 1 tablespoon olive oil and season to taste. Brush a chargrill with a little oil and heat on high until hot but not smoking. Cook salmon fillets for 3-5 minutes each side or until cooked to your liking.
Mix remaining wasabi with lime or lemon juice and remaining olive oil. Spoon mashed potato onto serving plates. Top with salmon. Drizzle dressing around edge of each plate and serve with lime wedges and broccolini.