For the Sweet & Sour Broth, place oil in a medium-sized pot and set over medium heat. Add grey mullet fillets and cook until browned, about 5 minutes. Add shallots, lemongrass, spring onions, garlic and ginger, and cook for 5 minutes. Add chicken stock and simmer to deglaze pan. Add fish sauce, tamarind puree, celery leaves, tomatoes, pineapple pieces and chillies, and bring to a boil. Reduce heat, add dill, coriander, salt and sugar, and simmer, covered, for 20 - 25 minutes.
Strain broth. Remove pineapple pieces and tomatoes from solids, and set aside for serving. Set aside remaining broth, keeping warm until serving. For the Fried Garlic Chips, heat oil in a small saucepan to 180°C. Add garlic and cook until golden and crisp, about 20 - 30 minutes. Remove from oil and set aside on a paper towel until serving. Alternatively, use store-bought garlic chips.
For the Lemongrass Oil, heat vegetable oil in a small saucepan set over low heat. Add the lemongrass, garlic and salt, and cook until lemongrass is golden, about 3 - 4 minutes. Remove the lemongrass and garlic, and set oil aside.
For the Vermicelli & Beansprouts, place vermicelli in a large pot with hot but not boiling water for 10 minutes. Strain vermicelli. Repeat the same steps for beansprouts. Add lemongrass oil and toss to coat the vermicelli and beansprouts. Set aside, keeping warm until serving.
For the Poached Grey Mullet Fillets, season fillets skin with salt. Place 2 - 3 tablespoons of oil in a large frypan and set over medium heat. Add fish, skin side down, and fry until cooked halfway through and skin is crisp, about 6 - 8 minutes. Remove from heat. Remove fish from the pan and set aside, skin side up.
Transfer the fish to the hot broth to reach halfway up the fillets. Stew at low heat until fish is cooked through, about a further 8 - 10 minutes. Remove from heat, keeping warm, until serving.
To serve, using chopsticks roll the Vermicelli Noodles in Lemongrass Oil into a tight bundle and place a bundle in the centre of each serving plate. Top with the reserved tomatoes, pineapple pieces, beansprouts and grey mullet fillets. Garish with fried garlic chips with coriander leaves, chilli slices and microgreens buckwheat. Add a little hot broth with lime juice and serve.