Step 1

Melt 2 tablespoons unsalted butter in a medium saucepan over medium heat. Add 2 tablespoons all-purpose flour and whisk until smooth and lightly golden in color, 1 to 2 minutes. Gradually whisk in 1 cup whole milk, about 1/4 cup at a time. Add 1/4 cup half-and-half, and whisking constantly, bring to a gentle boil. Cook and stir until thickened to sour cream-like consistency, about 2 minutes.

Step 2

Remove from the heat. Add1/2 teaspoon kosher salt, 1/2 teaspoon granulated sugar, 1/2 teaspoon ground mustard, 1/4 teaspoon pepper, and a pinch of nutmeg, and stir to combine. Add 3 tablespoons prepared horseradish and 1 tablespoon freshly squeezed lemon juice and stir to combine. Taste and season with more horseradish if needed.

Step 3

Let cool completely. Store in an airtight container until ready to serve. To serve, top with 1 tablespoon chopped chives, if desired.