Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion and garlic and cook gently for 5 minutes, until softened a little.
Add the chicken breasts and cook for 2-3 minutes on each side, until lightly browned.
Add the mushrooms and cook for 5 minutes until the juices have been released.
Stir the mustard and stock cube into the boiling water and stir until dissolved. Add to the frying pan. Stir well, then simmer, uncovered, over a medium heat for 10 minutes.
Turn the chicken breasts over, stir in the fresh tarragon and loosely cover with a lid or kitchen foil. Simmer for a further 10 minutes, or until the chicken is cooked and white throughout.Use a small sharp knife to cut into the thickest part of the chicken to check. There should be no pinkness and the juices should run clear.
Remove the frying pan from the heat and stir in the cream cheese until completely blended. If the sauce is too thin, continue to simmer, uncovered, for a little longer until the sauce has reduced and thickened to your liking. If the sauce is too thick, stir in a little water.
Taste and season with salt and black pepper, if needed.Garnish with a few fresh tarragon leaves (optional) and serve with an accompaniment of your choice.