One thing that Hanoi is not short of is pho joints. If you want to eat with the locals and be the only foreigner there, give Pho Cuong a shot.
I stumbled upon this place randomly on the way back to my hotel and ended up going back a few times. The pho is delicious and cheap. The shop is always full of locals starting from the early morning breakfast hours until the nighttime dinner hours.
Make sure you get a side of the fried dough sticks to dip into the broth.
Open for breakfast, lunch, and dinner. Cash only.