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 and turn the heat up to medium-high. Stir mushrooms every minute until mushrooms and onion caramelised. Season with salt and pepper halfway. This will take about 8 minutes.
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. About 1.5 minutes.
Reduce heat down to low and pour cream and dijon mustard in. Stir through, season with salt and pepper to taste. (note 1)
Mix cornstarch with 2 tablespoon of cold water well, pour into pan. Allow sauce to simmer for about 3 minutes, stirring regularly.
To serve, sprinkle finely diced fresh parsley or a sprinkle of fresh thyme leaves.