I ordered a "small" bun mam. Although Tuyết Mai's menu only lists a single price for its bowl of soups (the "small" size), a large size is also available. The smaller bowl is plenty for lunch, even for Generation XL types like me.
Bun mam is akin to a gumbo or a chowder; the broth is flavored with fermented fish paste (its tare, so to speak, to use a ramen analogy). Its toppings are primarily seafood, with the addition of pork of one form or another. The overall flavor profile is sour, spicy and fishy, in a positive way.
The word is that Mama Tuyết's "retirement" will include her hanging around the kitchen for another three years, as she was today and on my last visit. Bún mắm. thank you ma'am!
Where slurped: Tuyết Mai, formerly Ngoc Mai, 547 Hyde Street, San Francisco