Homemade mustard salad dressing is a great topper for any salad, but pairs particularly well with these crisp greens.
In a small bowl, mix mustard, lemon juice, shallots, salt, and pepper. Let stand at least 10 minutes.
Line a large, shallow serving bowl with the radicchio leaves.
Whisk oil into mustard mixture. In a bowl, pour dressing over frisée; mix gently, then mound on radicchio. Sprinkle with parsley.