Add the butter to a 12-inch skillet, and place over medium heat.
Once the butter melts, add the shallot, garlic and a pinch of salt and sauté until softened.
Add the flour to the skillet and stir to combine. Cook the flour/butter/shallot/garlic mixture for about 5-10 minutes, stirring frequently. You want the mixture to brown slightly, which will add color and flavor to the sauce.
Raise the temperature to medium high and add the stock to the skillet in a slow steady stream, whisking continuously to incorporate the stock into flour/butter/shallot/garlic mixture.
Whisking frequently, bring the sauce to a low boil to thicken. This should only take a few minutes.
Once the sauce is thickened, add the lemon zest, juice, and 1 tsp Kosher salt.
Add the capers, then season to taste.
Finish with the chopped parsley.
