Spray a large frying pan with low-calorie cooking spray and place over a medium heat. Add the onion and garlic, and fry gently for 5 minutes, until they have softened a little.
Add the chicken breasts and mushrooms, and continue to fry for 4 minutes, turning the chicken breasts over halfway through. The chicken breasts should be lightly golden.
Crumble in the stock cube and add the Prosecco. Simmer, uncovered, for 10 minutes. Turn the chicken breasts over. Add the cornflour to the cold water, and stir until smooth. Pour into the pan, stirring to combine. Add the thyme, then cover with a lid and simmer for a further 10 minutes — or until the chicken is cooked through, there are no signs of pinkness, and the juices 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 bit longer, until the sauce has reduced and thickened to your liking. If the sauce is too thick, stir in a little water.
Season to taste with salt and pepper. Then serve!