In a small saucepan, combine the vinegar, sugar, salt, and water and bring to a gentle simmer over medium heat. Place the onions in a heatproof bowl and carefully pour the hot liquid on top, pressing down with a spoon. Cool to a room temperature.
Add 1 inch of water to a pot with a steamer basket. Add the potatoes to the steamer basket and bring to a simmer. Cover and steam for about 15 minutes or until fork-tender.
Blend all the dressing ingredients with a whisk or immersion blender in a bowl and adjust the seasoning to taste.
To assemble the salad, divide the mixed greens, pickled onions, fennel, steamed potatoes, and smoked trout between 4 serving bowls and drizzle with the dressing.