What did I do wrong? I was convinced that the recipe said to use dashi - that stock used in Japanese cooking, made from kombu (kelp) and dried bonito flakes. After all, there would be lots of umami resulting from this. I thought it would be interesting to make my own dashi, so I followed Alton Brown's recipe for that.
The recipe actually called for chicken stock. Never mind! I also used more mushrooms than the recipe called for. I did follow the technique carefully - because it seemed unusual to me. I am glad that I did because the result was outstanding.