In a small saucepan, combine thinly sliced red onions, cider vinegar, granulated sugar, kosher salt, and a splash of water. Heat the mixture over medium heat until it begins to simmer, allowing the sugar to dissolve (about 2-3 minutes). Remove from heat and let cool.

In a mixing bowl, whisk together extra-virgin olive oil, reduced-sodium soy sauce, and white miso paste. Place the salmon fillets in the bowl, ensuring they are evenly coated. Marinate for 15 minutes.

In a small bowl, whisk together fresh lemon juice, honey or maple syrup, Dijon mustard, cayenne pepper, kosher salt, and black pepper. Slowly drizzle in remaining olive oil while whisking vigorously to emulsify.

In a large bowl, add lacinato kale and shredded Brussels sprouts. Massage the greens with half of the dressing for 2-3 minutes. Toss in fresh blueberries and chopped walnuts or almonds.

Preheat your broiler to high and line a baking sheet with parchment paper. Remove the salmon from the marinade, place skin-side down, and broil for 6-8 minutes.

To plate, divide the greens among serving bowls. Top with sliced pickled onions, a broiled salmon fillet, and diced avocado. Drizzle remaining dressing on top.