Combine onion, vinegar, sugar, salt, and 1/2 cup of water in a small saucepan over medium-high heat. Bring to a simmer and cook 1 minute. Remove from heat and let stand until ready to use.
Combine olive oil, soy sauce, and miso in a wide-rimmed bowl. Add salmon and gently toss to coat. Let stand 15 minutes.
Meanwhile, prepare Lemon-Cayenne Dressing by combining lemon juice, honey, Dijon, cayenne, salt, and black pepper in a small bowl. Gradually stream in olive oil, whisking constantly, until emulsified.
Place kale and Brussels sprouts in a large bowl and add half of dressing. Gently massage dressing into greens for 30 to 60 seconds until they start to soften. Add blueberries and nuts; toss to combine.
Place the top oven rack 6"-8" from broiler, and preheat broil setting to HIGH. Place the marinated salmon fillets on a foil-lined baking sheet, and broil for 6 to 8 minutes, or until just opaque in center. (If the salmon starts to burn, move it to lower rack.)
Assemble salad by dividing kale mixture evenly between each of 4 bowls. Drain pickled red onions and scatter evenly overtop. Top each salad with one miso-glazed salmon fillet, plus some avocado slices. Drizzle remaining dressing evenly overtop.