Hi. I lived in China 11 years and learned the language, and a lot of the cooking, mostly Northern style (heavier, savory flavors, lots of garlic and bean paste.)
Tian mian jiang means literally in Chinese "sweet flour paste." It is what is served in China with Peking Duck, not plum sauce, and not duck sauce. Good (read authentic) Chinese restaurants in the US serve it with Beijing Duck, too, but may call it plum sauce since that's what Americans are used to hearing. It doesn't have plums in it, though. Also, it's not spicy at all. It gives a deep, rich flavor to my favorite northern Chinese eggplant dish, which also has lots of garlic, a little sugar, and is finished with chopped coriander.
Tian mian jiang is a salty, savory paste made from ...give me a sec - I have a package of it I bought on a recent trip to China, I'm reading the ingredients as best I can...okay, in descending order: flour, water, salt, and a word with four components that I don't understand put together, breaks down like this: thick, shell, sour, sodium. Somebody who has studied cooking probably is jumping with their hand raised right now. Though there's no sugar listed in the ingredients, the fermented flour runny paste is remotely sweet. I guess that's why the word Sweet is in its name. Oh, mian means flour, and jiang means paste.
If I had to substitute for it, I'd use just a little miso, or any yellow bean paste that didn't have any extra flavorings like garlic or whatever in it, and I might add a pinch of sugar. The flavor is a bit similar - salty and pungent. Tian mian jiang's a bit runnier than bean pastes, so if you're substituting with something thick, you could loosen it with a tiny bit of water if necessary.
I do believe it is a northern flavoring; I've never seen or tasted it in southern areas in China. But I'm not the definitive authority on that, just telling you what I think I know! I'm going over to Tianjin again this Oct. I'll check with a friend who's from Hunan, a southern region with hot cooking. She's a great cook; she'll know. If you're still interested then....(if it's just a northern flavoring or used in the south.)
I have never seen tian mian jiang in America, but since I rarely live there, I can't say for sure whether or not it's available there; these days it probably is. I see you're in MD; when I'm in the States I live in Baltimore. Do you know that huge Asian grocery store out Rt. 50 heading west? That would be the place I'd look for it. I think it's called Hanareum...? Maybe at Rollings Road?
