A third volume on top reduced the gap still more.

Reduce to a simmer and cook until the volume of the liquid has reduced by half.

Boil until the volume of liquid has reduced by half.

Add the wine and cook until the volume of the liquid has reduced by half.

Simmer for 2-3 minutes, or until the volume of liquid has reduced by a third.

Add the walnuts and continue to simmer, or until the volume of liquid has reduced by another third.

Simmer for 4-5 minutes, or until the volume of liquid has reduced by about two-thirds.

Some moments passed and I heard the volume of the background noise reduced to silence.

Reduce the heat slightly and simmer for five minutes, or until the volume has reduced by a third.

Pour in the stock and simmer for 2-3 minutes, or until the volume of the mixture has reduced.