Belgian endive is a crisp, very pale leafy green with a slightly bitter flavor. It discolors after being cut, so prepare it no more than a half an hour in advance. This recipe is also suitable for Induction after the first two weeks.
Combine the vinegar, mustard, salt and pepper in salad bowl. Add oil a slow, steady stream, whisking until dressing thickens. This can also be done in a blender or by combining all ingredients in a jar with a tight-fitting lid and shaking vigorously.
Add the endive and radicchio and toss gently to coat. Divide among four plates and top with walnuts and blue cheese.