Toast the millet in a medium saucepan over medium-high heat, stirring occasionally, 3 to 4 minutes. Add 2 cups water and a pinch of salt and bring to a boil. Cover, reduce the heat to medium low and cook until the water is absorbed, about 15 minutes. Remove from the heat and let stand, covered, 5 minutes. Fluff with a fork, then let cool, uncovered, 5 minutes.
Meanwhile, make the dressing: Whisk the dijon mustard, vinegar, honey, mustard powder and a pinch each of salt and pepper in a small bowl until smooth. Whisk in the vegetable oil.
Chop the turkey, cut the tomatoes into small chunks and thinly slice the radishes. Combine the turkey, tomatoes and radishes in a large bowl. Add 2 to 3 tablespoons of the dressing, season lightly with salt and pepper and toss. Add the millet and all but 1/4 cup of the remaining dressing and toss again. Add 2 cups salad greens and toss.
Divide the remaining greens among plates. Top with the millet mixture and sprinkle with the cheese. Drizzle with the remaining dressing.