Heat a medium saucepan over medium heat; add 2 tablespoons of butter. When hot, add the mushrooms; sauté until they release their liquids and begin to lightly brown.
Add the shallot and cook for a minute or two until it starts to soften.
Add the tomato paste, stock, and juniper berries; bring to a boil.
Immediately reduce heat to a simmer and continue cooking until most of the liquid has evaporated.
Add the demiglace and fresh parsley and continue to reduce the sauce for 5 minutes or until it is thick enough to coat the back of a spoon.
Add goat cheese; stir until incorporated, then set sauce aside until nearly ready to serve.
While meat is resting (15 minutes), add aquavit to sauce and return to a simmer. Stir in the remaining tablespoon of butter (this is called “mounting”) to give the sauce extra shine and richness. Season with salt and pepper to taste.