Fantastic little restaurant run by a family of deaf people. Go for the Banh Khoai - crepes with fish and/or veg - but make sure you leave with one of their trademark bottle openers - handmade from a piece of word with a bolt on it. The owner will make, sign and date one for you. Amazingly effective and a memorable souvenir.
The atmosphere is really good, the staff friendly, the food is great and very cheap - a meal for less than £2. Good for veggie food too. Beware the imitators, including the place next door!
Lac Thien: 6 Dinh Tien Hoang. West of the river, near the Citadel.
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I've eaten at both and they're as good as each other in every way. Plus, if you do eat at both you get two bottle openers.