Add water to the porcini mushrooms and soak for about 30 minutes.
Peel and dice the onion and garlic.
Chop the sage and zest the lemon.
Add the garlic and onion into a hot pan greased with ghee and cook over a medium heat until translucent.
Stir occasionally to prevent burning.
Add the lemon zest and sage and cook until the onion is golden.
Pour in the bone broth, lemon juice and add the soaked porcini mushrooms with the water.
Add balsamic vinegar and mustard and bring to a boil.
Lower the heat and let it simmer until the amount is reduced by about half.
Be patient: if you don't reduce it enough, it will end up too thin and watery.
Add the cream and cook for just about 5 minutes.
Once done, take off the heat and season with salt and pepper to taste.
Pour in a jar if you used a shallow pan like I did and blend with a hand blender until smooth and creamy.
When done, serve immediately with meat, roasted vegetables or Creamy Keto Mash or let it cool down and store covered and refrigerated for up to 5 days.
Reheat as needed.