Using a mandoline, thinly slice the radishes, carrot and fennel bulb. Transfer to a large bowl of ice water and refrigerate for about 1 hour, until crisp.
Meanwhile, in a saucepan, bring the quinoa and water to a boil. Cover and cook over low heat until the water is absorbed and the quinoa is tender, 20 minutes. Let cool.
Drain and dry the vegetables. In a bowl, combine the lemon zest and juice with the oil. Add the quinoa and toss; season with salt and pepper. Serve the quinoa in bowls, topped with the vegetables.