Prepare the following, adding each to the same small bowl as you complete it: Finely grate the zest of 1 medium lemon until you have 1 teaspoon. Juice half the lemon until you have 2 teaspoons. Pick the leaves from 6 fresh parsley sprigs and finely chop until you have 2 tablespoons. Finely chop 1 medium scallion until you have 2 tablespoons. Finely chop 2 garlic cloves.
Melt 1 stick unsalted butter in a medium saucepan over medium heat. Add 1 tablespoon Dijon mustard, 1 tablespoon prepared horseradish, 1 teaspoon smoked paprika, ½ teaspoon kosher salt, and ¼ teaspoon cayenne pepper. Whisk until melted and combined. Add the parsley mixture and stir to combine. If it separates as it sits, stir to re-combine.
