Adjust oven rack to middle position and heat oven to 350 degrees. Scatter ½ cup skinned hazelnuts on rimmed baking sheet and bake until golden brown, 7 to 10 minutes, stirring nuts halfway through baking. Transfer sheet to wire rack and let cool for 10 minutes. Using bottom of dry 1-cup measuring cup, crush nuts into coarse pieces. Transfer nuts to bowl.

Line now-empty rimmed baking sheet with parchment paper and grease parchment. Add ¼ cup sugar, 2 tablespoons water, and ⅛ teaspoon table salt to medium saucepan and gently stir with silicone spatula until sugar is fully moistened. Lightly spray spatula with vegetable oil spray.

Bring sugar mixture to simmer over medium heat and cook without stirring until liquid is mostly evaporated and several large bubbles form without popping, about 4 minutes. Continue to cook, stirring constantly with greased spatula, until sugar mixture is amber-colored, about 2 minutes (mixture may smoke slightly; this is OK). Working quickly, stir in nuts; transfer nut mixture to prepared sheet and spread into even layer. Let cool completely, about 10 minutes. Chop candied nuts into rough ¼- to ½-inch pieces.

Whisk 5 tablespoons extra-virgin olive oil, 3 tablespoons white wine vinegar, 2 tablespoons honey, 1 teaspoon Dijon mustard, 1 teaspoon salt, and ½ teaspoon pepper together in large bowl. Add 1 large head radicchio, leaves separated and torn into 3-inch pieces; 1 thinly sliced Honeycrisp apple; and half of candied nuts and toss to combine. Season with salt and pepper to taste. Transfer mixture to platter, sprinkle with remaining candied nuts, and arrange 4 ounces blue cheese, chilled and sliced thin, over top. Serve.