Adjust the heat to medium, so that the mixture bubbles gently.

Stir, then let stand in a warm place until mixture bubbles and foams, about 10 minutes.

When mixture bubbles around edges and seems ready to boil, remove from heat.

Keep stirring while the mixture bubbles furiously for 2 minutes, then pour it into the smaller bowl.

Stir until smooth, then cook, adjusting heat so that mixture bubbles steadily.

Add the fish, and adjust the heat so the mixture bubbles but not furiously.

Return the saucepan to moderate heat and whisk until the mixture bubbles, about three minutes.

Let stand until the mixture bubbles, about five minutes.

Cook over sufficiently low heat so that mixture bubbles but mushrooms do not brown.

Reduce heat to low and stir until mixture bubbles.