In a large pan, add butter and onion and cook on medium heat for about 1.5 minutes or until onions softens.
Add mushroom slices, stir through mushrooms every minute until mushrooms and onion caramelised. Season with salt & pepper.
Reduce heat down to medium-low and stir in garlic for about 1 minute, until garlic is fragrant.
Deglaze the pan with stock, allow it to reduce by half. Reduce heat down to low and pour cream and dijon mustard in.
Mix cornstarch with some cold water well, pour into pan. Allow sauce to simmer and thicken lightly. Season again to taste.