First, in a pot on medium heat, add the butter and let it melt
Second ,add the finely sliced shallot and the mushrooms and stir frequently until they melt
Third, add the mustard and mix well
Then, add the meat bouillon cube with its water and stir frequently until the cube melts
Now, Pour the grape juice
In a separate cup, add the corn starch and the 2 tsp of water and stir well until the starch is fully dissolved
Now, add the dissolved starch to the pot and stir frequently until the sauce starts to thicken
Once the sauce thicken, let it cook for about a couple of minutes once it starts forming small bubbles on the surface
Finally, turn off the heat, your sauce is ready!