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, 1/2 teaspoon kosher salt, and 1/4 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.