Not traditional, but I love Momofuku's pork buns! A bit pricey at $9 for two. I've discovered I can make a healthier version. The dim sum store on Pell Street sells the exact same buns Momofuku uses. It's called steamed pita bread. I then insert some roast pork or BBQ chicken, pickled veggies, cilantro and Sricha sauce. Not the same fatty porky goodness as the Berkshire pork belly Momofuku uses, but a much better everyday option.
I don't remember if the baked Char Su Bao is onion free but the place on the corner of Canal Street (and the block that runs behind the Tombs, I forget the name) has been our family favorite for over 30 years. Also try their coconut buns (also baked) and for the steamed buns try the combination buns. The restaurant/bakery is right off the triangle that runs in the middle of canal street and is directly across from the tourist booth.