Begin by poaching the salmon. Bring water to the boil in a large saucepan and add matcha, chilli flakes, and peppercorns. Whisk well, then reduce the heat to low. Leave to infuse for 4 minutes, then stir in the mirin. Place salmon in the liquid, skin-side down. Cover and cook over a low heat until salmon is just cooked and still slightly translucent in the middle (around 4–5 minutes – salmon will continue to cook after being removed from heat). Remove pan from heat and set aside.