Boil water in a kettle. Squeeze the lime juice into a cup, stir in the sugar until dissolved. Stir in the fish sauce and toasted sesame oil. Shred the mint. Place the noodles in a bowl and cover with boiling water. Leave while you prepare the fish. Crush and mix the salt, thyme, cayenne and black pepper together. Tip on to a plate. Pat the fish dry with kitchen paper and press the top side of the fillet into the salt. Heat the vegetable oil in a frying pan and when hot, place the fish, salted side down, in the pan. Press with a spatula for about 30 seconds to sear, reduce the heat, cook for 30 more seconds, then slide a spatula under the fish to turn. Cook for a further 2-3 minutes, depending on the thickness of the fillet, until just cooked through. Drain and shake the noodles dry, return to the bowl and toss with the lime sauce and mint. Serve the fish, blackened side up, over a mound of noodles.