Preparation Crumble yolk into a blender, then add vinegar, mustard, salt, and pepper and blend until combined well. With motor running, add oil in a slow stream, blending until combined well. Toss mâche with vinaigrette just before serving. Cooks' note:Vinaigrette can be made 1 day ahead and chilled, covered. Bring to room temperature, then whisk until combined well.
Preparation Crumble yolk into a blender, then add vinegar, mustard, salt, and pepper and blend until combined well. With motor running, add oil in a slow stream, blending until combined well. Toss mâche with vinaigrette just before serving. Cooks' note:Vinaigrette can be made 1 day ahead and chilled, covered. Bring to room temperature, then whisk until combined well.