I had a hard time developing a good substitute for hoisin sauce, which is made from fermented soybeans, garlic, sugar, salt, and spices. I wanted to replicate the taste and texture by using other ingredients that might be on hand. I started with American barbecue sauce as the base, then added soy sauce, molasses for thickening and sweetness, and five-spice powder for flavor. That mixture came fairly close to hoisin but was missing a key component: the funky sourness of fermented soybeans. I turned to another food that has such a flavor: yeast extract (Marmite/Vegemite). A little yeast extract did the trick and made the homemade hoisin sauce a good substitute for the real thing. I realize that many home cooks won't have yeast extract on hand; but for those that do, the recipe makes a fair approximation. (The recipe is in the book; if anyone's interested, I'll post it here). In a pinch, you can also get away with using Chinese brown bean sauce or Chee hou sauce, although both of these are spicier and less sweet than hoisin. You could add a little sugar to up the sweetness.
What didn't work? Finding a replacement for foie gras. I left foie gras out of the book entirely. Although, a good-quality liver pate can stand in for pate de foie gras.