1. Place mustard, vinegar, salt, and pepper in a medium bowl, and whisk to combine. Slowly drizzle in walnut oil, whisking constantly. Add cranberries, and stir to combine. Set dressing aside.

2. Core apples; cut them into 1/4-inch-thick slices, then cut slices into 1/4-inch-thick spears. Place in a medium bowl with lemon juice. Cut endive in half lengthwise, and remove core. Cut into 1/4-inch-thick spears, and place in bowl with apple mixture. Add yellow pepper; combine.

3. Arrange frisee on a serving platter. Add the dressing to the endive mixture, toss to combine, and arrange on top of frisee. Serve immediately.