In a small saucepan, combine 50ml soy sauce, 50ml mirin, 50ml sake, and 50g sugar. Cook over low heat, stirring occasionally, until the sugar dissolves and the sauce thickens slightly. Set aside to cool.
Cut 500g of salmon fillets into 4 equal portions. Pat dry with a paper towel and season with a pinch of salt and black pepper.
Heat 2 tablespoons of oil in a non-stick pan over medium heat. Place the salmon fillets skin-side down and cook for 3-4 minutes until the skin is crispy. Flip the fillets and cook for another 3-4 minutes until the salmon is cooked through.
Reduce the heat to low and pour the prepared teriyaki sauce over the salmon fillets. Cook for 1-2 minutes, spooning the sauce over the fillets to coat them evenly.
Remove the salmon from the pan and transfer to a serving plate. Drizzle with the remaining teriyaki sauce and garnish with 1 tablespoon of toasted sesame seeds and 2 tablespoons of chopped spring onions.