on malaysia side, after the complex bebas cukai, just next to the immigration complex, theres are few stalls selling groceries items from thai.

behind it, there is a food stall. the laksa siam there is nice & delicious.
you will be given a bowl with just laksa and its gravy. there is also another large coaster with condiments in it (cucumber, chilli, beansprout, tofu etc - add in as you like). other menu are also quite good