Preheat oven to 400° F. Line a large cookie sheet with parchment paper; set aside.
Heat olive oil in a large skillet over medium heat. Add onion and cook for 5 minutes. Reduce heat to low and continue cooking for about 30 minutes or until onion is caramelized, stirring frequently. Add garlic. Sprinkle with salt and pepper. Cook for 2 minutes; remove from heat. Drain and set aside. Wipe skillet clean with a paper towel.
Using the same skillet, melt butter over medium heat. Add apples and sprinkle with sea salt and crushed red pepper. Cook for 8 to 10 minutes just until apples are softened, gently turning often. Drizzle apples with maple syrup and continue cooking for 3 to 4 minutes, until apples are glazed. Remove from heat; set aside.
Meanwhile, place kale in a large bowl and drizzle with olive oil and sprinkle with sea salt. Use hands to toss kale to coat evenly with oil.
In a food processor, purée olive oil, apple cider vinegar, smoked blue cheese and Italian seasoning. Place flatbreads on parchment-lined cookie sheet. Spread sauce evenly on flatbreads and top each with kale then onion mixture. Sprinkle each with blue cheese and top with a layer of apple slices. Sprinkle walnuts over each flatbread. Bake for 12 to 15 minutes or until flatbreads are golden brown and cheese is melted. Serve immediately.