Traditional English Parsley Sauce

Ingredients

• Steps to Make It
• Gather the ingredients.
• In a medium-sized saucepan, melt the butter over medium heat.
• Stir in the flour and mustard (if using).
• Stir thoroughly to form a thick paste.
• Cook gently for 2 to 3 minutes more, watching the heat to ensure the paste does not burn. This will cook off the taste of raw flour in the finished sauce.
• Gradually whisk in the milk.
• Bring to a boil, lower the heat, and simmer for 5 minutes, frequently whisking to make sure there are no lumps. The sauce should be quite thick but still pouring consistency. If too thick, add a little more milk.
• Add the fresh parsley and stir well. 
• Season with a good pinch of sea salt and a few black pepper grinds. Taste and add more as needed.
• Keep the sauce warm, on low heat, until needed. If keeping sauce warm for more than 15 minutes, laying a piece of buttered parchment paper on the surface will prevent a skin from forming. If a slight skin does form, whisk thoroughly over heat, and it should disappear.
• Serve and enjoy.
• Tips
• If the sauce is too thick, whisk in milk a little at a time until the desired consistency is reached. If the sauce is too thin, whisk a small amount of flour with a little water to create a slurry. Whisk into the parsley sauce over heat and cook until thickened.
• Dried parsley is not a good substitute for the fresh herb, particularly in this recipe. Like cilantro and some other leafy herbs, parsley loses most of its flavor when dried and will result in a bland sauce.
• 2 tablespoons/25 grams butter
• 2 tablespoons/20 grams all-purpose flour
• 1 1/2 teaspoons English mustard, optional
• 1 cup/250 milliliters milk, more as needed
• 1 handful fresh flat-leaf parsley, chopped
• Sea salt, to taste
• Freshly ground pepper, to taste