Breast meat is lean and unforgiving of high heat. Poach it barely below a simmer and it stays juicy without any fat at all.
Stock, bay and salt in a pan wide enough to hold the breasts in one layer. Bring it up and then back it off. At 80 °C the surface is completely still — no bubbles, no tremble, just steam. If you can see movement, it is too hot.
They should be submerged. Fourteen to sixteen minutes for 180 g breasts, holding the temperature. Adjust the flame constantly; this is fifteen minutes of low-grade attention rather than fifteen minutes off.
The breasts finish here on residual heat. Pull one out and check: 65 °C at the thickest point. Not 72 — that is the thigh number, and breast at 72 °C is already dry.
A food processor turns salsa verde into a bruised green purée. Chop the herbs, capers, anchovy and garlic together on a board until fine but still distinguishable.
Into a bowl with the vinegar and mustard, then beat in the oil last with a fork. It should be loose and spoonable rather than emulsified.
Two-centimetre slices. Salsa over, and a spoonful of the poaching liquid too — it is now a light, well-seasoned stock and it stops the plate being dry.
Keep the rest of that liquid. It has picked up flavour rather than lost it.
Use the surface as your instrument. At 80 °C there is steam but the water is glassy and completely still. The first small bubbles rising from the base put you around 85 °C, which is the outside limit. A visible tremble across the surface is 90 °C and already too hot.
For doneness without a thermometer: press the thickest part. Raw breast is soft and yielding, cooked breast springs back firmly, and overcooked breast is hard. The middle of those three is what you want, and it takes a few attempts to learn.