Spray a small saucepan with low calorie cooking spray and place over a low heat.
Add the onion and fry for 5 - 10 minutes. Stir until golden and slightly softened.
Add the garlic, mushrooms, mustards and chicken stock. Simmer over a low heat, uncovered, for 15 - 20 minutes.
Remove the pan from the heat and stir in the cream cheese until completely blended.
Season to taste with black pepper.