Mix water with the fish sauce in a small bowl and set aside.
Combine shallots, garlic, black pepper and chilli flakes in a separate bowl and set aside.
Heat water and sugar in a large frypan over medium heat stirring occasionally until sugar turns deep golden brown. Gently stir in the fish sauce mixture and bring to a boil.
Stir in the shallot mixture and cook until shallots soften then add the catfish. Cover and cook the catfish until the fish flakes easily with a fork; about 5 minutes on each side. Place catfish on a large plate, cover and set aside.
Increase heat to high and stir in 1/2 teaspoon of sugar. Stir in the lime juice and any sauce that has collected on the plate. Bring to a boil and simmer until the sauce has reduced. Pour sauce over the catfish and garnish with spring onions and coriander.