Make the Dressing:
Zest the lemon into a small bowl and then squeeze to add the juice. Whisk in the Dijon mustard. Then, gradually whisk in the oil until a smooth emulsion forms. Season to taste with salt and freshly cracked pepper.
Assemble the Salad:
First, peel the rounded side of the celery with a peeler to remove the strings. Then, thinly slice the cauliflower, celery, radish, and onion on a mandoline into 1/8-inch thick slices. Alternatively, you can use a food processor with a slicing blade.
Soak the red onion in ice water for 10 minutes to soften its bite. Drain and pat dry. Slice the apple into 1/8-inch slices before gently tossing the ingredients together in a large bowl with the parsley.
Drizzle in the dressing and use your hands to toss everything until it’s evenly coated. Use a vegetable peeler to shave wide, thin slices of manchego over the salad. Be generous. Then, crumble the walnuts in your hand as you sprinkle them all over the salad. Season with a pinch of sea salt and freshly cracked pepper before serving.