Make the Honey-Balsamic Walnuts:

1. Line a baking sheet with parchment paper or a silicone baking mat.
2. In a small skillet set over medium heat, stir together sugar, honey, olive oil, balsamic vinegar and cayenne until sugar is dissolved. Continue cooking, stirring frequently, for 2-3 minutes or until the syrup begins to bubble and thicken.
3. Stir in walnuts. Cook, stirring constantly, for 5-6 minutes or until the walnuts are coated with a thick and sticky glaze.
4. Turn out onto the lined baking sheet. Using a silicone spatula, spread the walnuts out into a single layer, separating into individual pieces as much as possible. Let cool completely, then transfer to an airtight container lined with paper towel. Walnuts can be prepared up to three days in advance.

Prepare the Dressing:

1. In a small mixing bowl, whisk together buttermilk, sour cream, mayonnaise and vinegar until combined. Crumble blue cheese into the bowl, and stir until blue cheese is incorporated. Adjust the seasoning with salt and pepper to taste. Refrigerate until ready to use, preferably for at least 24 hours. Dressing can be made up to three days in advance.

Broil the Steak:

1. Preheat the broiler (or grill). Pat the flank steak dry using paper towel. Sprinkle both sides with salt, then press in the cracked peppercorns to make a crust. Transfer to a broiler pan and place under the preheated broiler to cook for 4-5 minutes per side, until cooked to medium-rare or a meat thermometer registers an internal temperature of 165F. Tent with aluminium foil and set aside to rest for 5 minutes.

Assemble the Salads:

1. While the steak is resting, prepare the salad greens. Tear radicchio leaves into bite sized pieces, discarding the core. Strip the endive leaves off the core, and chop into inch-long pieces; discard core. Combine radicchio, endive and the remaining greens in a large bowl.
2. To assemble, toss salad greens with 1 cup of blue cheese dressing until evenly coated. Divide the greens across six small salad plates (or four large dinner plates). Top each salad with a scattering of honey-balsamic walnuts and crumbled blue cheese.
3. Cut flank steak into thin slices across the grain. Arrange a few slices onto the top of each salad. Serve immediately.