Melt a tablespoon of butter in a medium saucepan over a low/medium heat. Add the onion and sweat gently for a few minutes until tender. While the onion is cooking, thinly slice the mushrooms and crush the garlic.
Add the mushrooms, garlic, dried herbs, mustard, remaining tablespoon of butter, and orange juice to the saucepan. Stir well to combine. Cook gently for 4-5 minutes to brown the mushrooms all over.
Add the vegetable stock to the saucepan. Stir well to combine. Bring the stock to a gentle boil, then reduce to a simmer. Simmer the mixture for five minutes to combine the flavors, soften the mushrooms, and reduce the sauce. You may add salt and pepper to taste as desired. This will depend on the strength of seasoning already in your stock.
Sprinkle the xanthan gum over the gravy and immediately whisk in. Stir well to combine until the powder is completely dissolved. Continue to stir and simmer gently until the gravy has thickened. You can add more or less gum depending on how thick you like your gravy. Remove from the heat and serve immediately over your Keto Christmas mains.