Start by soaking the quinoa to remove its bitter aftertaste; place it in a bowl of cold water for 30 minutes, then strain and rinse under running water. Drain and cook according to package directions, about 15 minutes.
Meanwhile, prepare the dressing. Place garlic cloves, olive oil, lemon juice, and honey in a blender. Season with salt and pepper and blend until smooth.
When the quinoa is cooked, fluff with a fork and set aside to cool. Slice the radishes, cucumber, and onions very thinly. Drain the feta and cut into cubes.
Place all the ingredients in a bowl, add the dressing, and mix thoroughly. Serve immediately or store in the refrigerator for up to 5-6 hours.