Why is my sauce bitter?

The caramel burnt. You need to take the sugar off the heat the moment it turns amber. If it goes black, it's burnt and the bitterness can't be removed.

What fish can I use if I don't have catfish?

Pangasius, carp or tinned tuna. Firm white fish with tight flesh holds its shape. Salmon works too, but simmer it for less time.

Do I need a clay pot?

No, but a heavy-based pot holds heat evenly and stops the sauce catching. If you only have a thin pan, simmer at a lower temperature and stir more often.

How long does cá kho tộ keep in the fridge?

Four days, and the flavour deepens each day. The sauce, with fish sauce and caramel, doesn't spoil quickly. It's best on the second day.

What can I use instead of coconut water?

Water with a tablespoon of coconut milk, or just water with a tablespoon of sugar. The flavour will be flatter, but the sauce will thicken the same. Coconut water adds a mild sweetness and mineral note.

How much rice per serving?

Seventy to eighty grams of dry jasmine rice per serving. It roughly doubles in volume when cooked. Serve with a piece of fish and two tablespoons of sauce.