Roughly chop <quantity>1</quantity> large bunch of flat leaf parsley and discard stems. Put leaves in a blender

Add <quantity>½<unit>cup</unit></quantity> of water and <quantity>½<unit>cup</unit></quantity> of butter to a saucepan over medium heat. Warm it until the butter is completely melted, and add the mixture to the blender

Blend on high speed for 5m until the mixture is bright green and the parsley is mostly broken down

Pass through a fine sieve into a wide container. Gently press to extract the maximum amount of sauce

Place it in the refrigerator for 2h to cool down

Season with salt to taste. Add <quantity>½<unit>tbsp</unit></quantity> of apple cider vinegar and <quantity>½<unit>tsp</unit></quantity> of mustard. Stir until fully incorporated. The parsley sauce is ready

Wash and dry <quantity>1</quantity> small head of a romaine lettuce. Remove the outer leaves and any damaged leaves. Cut the lettuce in half

Place each half on a plate. Generously cover them with <quantity>1<unit>tbsp</unit></quantity> to <quantity>2<unit>tbsp</unit></quantity> of parsley sauce. Finish with salt to taste

Place <quantity>4</quantity> ground ivy leaves on top of the lettuce. The lettuce with parsley sauce is ready

Enjoy it in a sunny day!