Mushroom substitutes I use mushrooms mainly to bulk up food, for example, I add them to cottage pie, lasagne, etc.. and they can turn 1lb of minced into 1.5lb.  Also, I particularly like venison stew (which seems to require a huge quantity of mushrooms).  

The problem that I have is that my daughter can't stand mushrooms, and studiously picks them ot of all her food.  Is there an alternative for them for the two uses above?
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.