I noticed a mao cai menu at Huiyuan Noodles but hadn't realized it was from their sister restaurant a few doors down. The staff cover both places, and you can sit in either restaurant and order from the other as much as you like.
Their vegetable selection is fresh (though I would like to see pumpkin and a few more green things). They do have lots of meats to cook in the very good, complex flavoured soup: chicken, chicken heart, chicken gizzard, beef, beef stomach, pork throat, crispy fried pork, goose intestine, etc, etc. They also add a spoonful of preserved black beans (豆豉） so you stir things up before eating. It's a little more expensive than most mao cai places, but very worth it.
My vegetarian mao cai, which came out with a few shreds of beef in it, before being stirred up:
Once when I found a bug in my dish they apologized and cooked up a fresh order (after assuring me that kind of bug wasn't poisonous), and comped my one yuan bowl of rice. I won't hesitate to go back.
Chinese name: 惠元冒菜
Address: 红瓦寺街 共和村