What is the basis for the rich, pinkish Sauce Nantua served with Quenelle de Brochet?

Answer

A creamy crayfish reduction

Sauce Nantua is traditionally prepared as a rich, pinkish reduction made from crayfish, which requires patient preparation to extract flavor from the shells.

What is the basis for the rich, pinkish Sauce Nantua served with Quenelle de Brochet?
foodLyoncuisinedishbouchon