Tempering a custard

How to tell it is going right

• The hot liquid went into the eggs a little at a time with constant whisking, and only then did everything go back to the pot.
• The heat under it stayed gentle throughout and it was never allowed to boil.
• Stirred continuously and right into the corners of the pan, where it thickens first and sets unnoticed.
• Judged by behaviour: it thickens enough to coat the back of a spoon and hold a clean line when a finger is drawn through it.
• Taken off before it looks finished, and strained. Residual heat carries it the rest of the way, and a custard held on the heat until it looks right arrives past right.

Where it goes wrong The hot liquid went in too fast, or the pot went back over full heat to save time. The yolks set into threads before they could thicken smoothly, and a scrambled custard does not come back; straining hides a little of it and nothing hides the rest.