In a medium saucepan, add the butter, shallot and mushrooms. Cook over medium heat for 7-9 minutes until the mushrooms are softened and have released most of their liquid.
In a liquid measuring cup or small bowl, whisk together the milk and cornstarch until smooth.
Pour the chicken stock and all the spices into the saucepan. Bring to a simmer.
While whisking vigorously, slowly pour in the milk/cornstarch mixture into the simmering stock, stirring constantly.
Continue to stir and cook until the mixture bubbles and thickens, about 3-4 minutes.
Remove from the heat and season to taste with more salt and pepper (I usually add a pinch more of each!). Use immediately in a recipe or cool and refrigerate for up to a week.
