Rinse the quinoa through a fine sieve. Add the quinoa and 1 3/4 cups water to a medium sized saucepan. Bring the water to a boil. Reduce the heat to low, cover, and simmer for 13 minutes. Remove the quinoa from heat, fluff gently with a fork, and re-cover. Allow the quinoa to steam for 5 minutes.
To prepare the salad dressing, whisk together all ingredients in a small mixing bowl or liquid measuring cup.
To prepare the salad, mix the quinoa, radishes, fennel, arugula, and nuts together in a large mixing bowl. Add the dressing and toss to combine. Season the salad to taste with extra salt and pepper, and toss again. Divide the salad onto four plates, and top each plate with a quarter of the tofu. Serve.