Spray a large frying pan with low-calorie cooking spray and place on a medium heat. Add the onion and garlic, then fry gently for 5 minutes – until softened a little.
Add the mushrooms and cook for 8-10 minutes – until the onions and mushrooms are caramelised and lightly golden.
Add the chicken breasts, and cook for 2-3 minutes on each side – until lightly browned. Then pour in the chicken stock, and simmer over a medium heat for 10 minutes. Turn the chicken over and partially cover your pan with the lid. Simmer for a further 10 minutes – or until the chicken is cooked through. You'll know the chicken is fully cooked when it shows no sign of pinkness, and the juices run clear.
Stir in the red pesto until completely mixed. Remove from the heat, then stir in the reduced fat cream cheese – until completely blended.
If the sauce is too thin, keep simmering, uncovered – until it's reduced to your liking. If the sauce is too thick, add a splash of water to your pan, then stir.
Stir in the spinach leaves until slightly wilted, then season to taste with salt and black pepper. Serve with a small portion of basmati rice, or another accompaniment of your choice.