Step 1

Melt 4 tablespoons unsalted butter in a small saucepan over medium heat. Add 6 tablespoons all-purpose flour and cook, stirring constantly, until it forms a smooth paste, 1 to 2 minutes. Remove from the heat.

Step 2

Chop 1 pound white button or cremini mushrooms into bite-sized pieces. Place in a large skillet and add 1 1/2 cups of the broth. Cook on high heat until the mushrooms have softened but still hold their shape, and almost all liquid is evaporated. Add the remaining 1/2 cup broth. Once boiling, stir in the flour paste and cook until thick enough to coat the back of a spoon. Remove from the heat and let cool 15 minutes.

Step 3

Add 8 ounces sour cream (about 1 cup) and stir to combine. Taste and season with kosher salt and freshly ground pepper as needed.