Toast the sunflower seeds in a dry nonstick pan. Remove from the heat, and place on a plate to cool.
Rinse, trim, and thinly slice the radishes. Rinse and finely chop the scallions. Rinse and trim the arugula, then tear into bite-sized pieces if necessary.
Mix the lemon juice with the olive oil, then season to taste with salt and pepper. Mix the prepared ingredients with the dressing and toss to combine. Arrange the salad on plates.
Crumble the feta cheese over the plates, top with the capers, and serve.