Simmer mirin and sake in a saucepan over high heat until reduced by a third (2-4 minutes). Add soy sauce and bonito flakes, stir to combine and set aside.
Heat half the sesame oil in a frying pan over medium-high heat, then cook garlic until golden-brown and slightly crisp (3-4 minutes). Transfer garlic to mirin mixture and refrigerate in a non-reactive airtight container for up to 3 days.
Trim tuna into a rectangle. Heat remaining sesame oil in a frying pan, sear tuna on all sides until just golden (30 seconds to 1 minute), transfer to mirin marinade and refrigerate to marinate (10-15 minutes).
For wasabi and chilli bean tartare sauce, bring a saucepan of water to the boil, add eggs and cook until soft-boiled (5 minutes). Run under cold water, peel and process in a blender with onion until smooth (1-2 minutes). Add mayonnaise and parsley and process until smooth (1-2 minutes). Halve egg mixture and add wasabi to one half and chilli bean paste to the other, and stir to combine.
Remove tuna from marinade, slice across the grain to 2cm-3cm thick and arrange on plates. Spoon a small amount of each sauce over tuna slices, scatter with micro-cress and serve.
