Sour noodles from Lạng Sơn (phở chua Lạng Sơn)

I never heard about this food until yesterday! The shop located in small and quiet alley in dist. 3. We went there around 4:00 PM, few customers behind the stall! But the waiter told us - school children, student love it - sour noodles!
Ok, let's try and we ordered three bowls.

Sour noodles with chicken from far land in the North Lạng Sơn! 

I'm very curious why sour? why Lạng Sơn (Lạng Sơn is a far northern province, near to borders with China!) and I'm living in Sài gòn, in South, over thousand km in distance!
I've never been there and probably in the near future I won't have a chance to travel to discover any specialities in that area. So I'm happy to do it here in my home town!

and mix all together! 

This noodle served with cooked chicken, peanut, grated fresh water spinach (rau muống bào), fried shallot, Vietnamese mint, chili sauce and some secret "sour and sticky sauce"! The taste is sweet, sour, a bit spicy ...to me it's similar to some chicken salad but with noodles!

Pork rinds with chili to add to the noodle

If for beef noodles or some other rice noodles we may add lemon juice, black soy sauce ...but for this sour noodle: pork rind (tóp mỡ) mixed with chili! Something new!

Anyway it's a lovely discovery, some taste from far away land in Sài gòn!

In case you want to try, the add is on the photo below! Good try!

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Here's the stall at 242/101 Nguyễn Thiện Thuật, dist. 3!