Mushroom substitutes
I concur with Elendi's answer, especially about the dumplings in stew (but I love dumplings).

As far as stretching minced (ground) meat, I recommend you try TVP. You might be amazed how innocuous it is, and how far you can stretch meat. For an organic product that does not contain hexane (the potential carcinogen noted in the Wiki article) try TSP. It's a bit more expensive, but once hydrated, it's a whole lot less expensive than meat.