Melt 2 tablespoons unsalted butter in a medium pot over medium-low heat.
Add 2 tablespoons of all-purpose flour and stir for 1 to 2 minutes, or until smooth and light golden.
Add 1 cup of whole milk, 1/4 cup at a time, whisking carefully. Add 1/4 cup half-and-half and bring to a moderate boil, stirring constantly.
Stir and simmer for about two minutes, or until it reaches a sour cream consistency.
Take it off the heat. Add 1/2 teaspoon kosher salt, 1/2 teaspoon sugar, 1/2 teaspoon ground mustard, 1/4 teaspoon pepper, and a touch of nutmeg.
Mix in 3 tablespoons of prepared horseradish and 1 tablespoon freshly squeezed lemon juice. If you believe it requires more horseradish, taste it and add more.
Allow to cool completely. Store in an airtight jar until ready to serve. If desired, garnish with 1 tablespoon minced chives before serving.