Step 1Grate the zest from the oranges and reserve. Peel the oranges and cut them into one-fourth- to one-half-inch crosswise slices. Cut each slice into quarters and set aside. You should have about 2 cups orange pieces.
Step 2Thinly slice the red onion crosswise into roughly one-fourth-inch sticks; you should have about one-third cup. Soak in cold water to reduce the sting.
Step 3Place the quinoa in a strainer and rinse under running water until the water runs clear, 1 to 2 minutes. Drain well, and turn the quinoa into a medium saucepan and toast over medium heat, stirring constantly, until it smells nutty and turns a light golden color, about 5 minutes.
Step 4Add the water, the reserved orange zest and one-fourth teaspoon salt. Bring to a slow simmer, cover and cook until the quinoa is dry, 20 to 25 minutes. Remove from the heat and set aside 10 minutes before fluffing gently with a fork. Cool to room temperature.
Step 5Turn the quinoa into a mixing bowl and gently stir in the red onion, serrano pepper, arugula and walnuts. Drizzle over the olive oil, stir gently and season to taste.
Step 6Mound the quinoa mixture on a serving platter and arrange the orange slices on top. Scatter the radishes over the oranges and then the crumbled feta. Serve at room temperature.