Make the quinoa and tuna - In a saucepan on medium heat, add quinoa. Continuously stir until it begins to toast and smell nutty, about 5 minutes. Add the water and salt, then bring to a slow simmer. Cover and cook until water is absorbed about 30 minutes. Allow to cool.
In a bowl, add tuna, half avocado, mayo, salt, and pepper. Mix, but keep slightly chunky. Set aside.
Make the dressing and salad - In a small bowl, whisk mayonnaise, miso, vinegar, soy sauce, rice vinegar, and oil.
In a large bowl, add cabbage, carrots, scallions, and radishes. Toss about ¾ of dressing in salad and mix together to combine.
To assemble - In a new bowl, add half of the quinoa, half the dressed salad, and tuna salad. Top with slices of half an avocado. Drizzle remaining dressing, and enjoy.