The sauce: make this at least one hour before cooking the chicken
Sweat the eschalots with butter and flour for a few minutes, then add verjuice. Slowly add the stock, and simmer for 10 minutes with vanilla beans and saffron.
Remove from the heat and pass through a fine sieve into a clean pan.
Place the sauce over a low heat, stir in the tomato paste and rice vinegar until thoroughly blended. Slowly add the cream, taste and season.
Remove from the heat. Split the vanilla bean, scrape the seeds and return them to the sauce.
Replace the scraped-out pod in the sauce and leave to infuse for one hour.
Gently reheat before serving, and remove spent pod.
To cook the chicken, fill a pan with water, add the allspice and bring to a simmer.
Add the chicken breast fillets in a single layer, making sure they are completely covered by the water, bring up to boiling point then immediately reduce heat to a simmer. Poach gently for 12-15 minutes until tender, moist, and just whiter than pink when cut through the thickest part.
Remove, drain and set aside, discarding stock.
To serve, spoon enough vanilla sauce on to the warmed plates to cover the base. Place mashed potato or white beans in the centre of the plate, top with chicken. Drizzle extra vanilla sauce over the top, then place two or three cooked asparagus spears on the chicken.
Although vanilla beans can sometimes be rinsed and dried for re-use, in this recipe the beans are used to the full and are ready for disposal once the sauce is made.