Preheat oven to 180C/350F on fan bake.
Scatter walnut pieces on a baking tray and place in the oven for 8-10 min until the walnuts are starting to brown and you can smell their aroma. Remove and allow to cool slightly.
Reserve 1/4 cup of toasted walnuts for the salad. Place remaining 3/4 cup of walnuts in a blender or food processor with 1 Tbsp oil, pinch of sea salt and freshly cracked pepper. Process until a gritty paste. You still want some texture to it, not completely smooth. Smear on the bottom of your serving dish or bowl.
Slice the witlof leaves in half crossways then arrange with the rest of the salad ingredients on the walnut butter. Scatter over remaining toasted walnuts.
To make the dressing, whisk together mustard, honey and lemon juice in a small bowl. Slowly drizzle in oil while continually whisking to create an emulsion. Season with sea salt and freshly cracked black pepper to taste.
Drizzle the dressing over the salad and serve. Best eaten on the day.