Melt 3 Tbsp. butter in a Dutch oven over medium-high heat; add mushrooms, and sauté 10 to 12 minutes or until liquid evaporates. Transfer to a bowl.
Reduce heat to medium. Melt remaining 5 Tbsp. butter in Dutch oven. Whisk in flour until smooth; whisk 1 minute. Gradually whisk in cream and next 2 ingredients. Cook, whisking constantly, 2 minutes or until melted and smooth. Remove from heat; stir in mushrooms. Use immediately, or cool completely. Freeze in 1 1/4-cup portions in plastic freezer bags. Thaw before using.
Note: We tested with Knorr Homestyle Concentrated Stock.