Bring the chicken stock to a boil and then reduce to a simmer. Reduce the stock until it reaches 500ml.
In another large saucepan, heat the cream and butter together over medium heat. Whisk frequently to prevent the bottom from scorching.
The cream will split, but keep cooking. Once it turns brown and smells amazing, turn off the heat.
Add the onions, garlic and sage to the pan. Turn the heat back on to low and cook gently until soft (about 3 minutes).
Add the flour and continue to cook for another 2-3 minutes on low, stirring frequently with a spoon
Turn the heat off and allow the roux to cool for about 2 minutes.
With a whisk ready, slowly pour in the hot chicken stock (measured out to 500ml). Whisk to remove any flour lumps.
Add the rest of the ingredients and blend until smooth.
Adjust the seasoning to your liking. Keep the sauce in a small saucepan with a lid and bring to a boil before serving. Enjoy!
If you are using a stock that is already seasoned with salt, you may need to add less or no salt. Check the seasoning before you add extra salt.If your stock is already salted, don't reduce the stock and just use 500ml of stock instead. If the sauce is too thin add more chicken stock. If the sauce is too thick continue cooking it longer until it thickens. (Adding raw flour to the sauce will make it clump up and add an unpleasant flour taste).