Cut tenderloin crosswise into four medaillons. Place between two layers of plastic wrap. Using a meat mallet, pound turkey medaillons to about 1/4-inch thickness. Uncover medaillons. Cover medaillons evenly with sliced lemon grass, serrano pepper slices, and salt. Cover with a new piece of plastic wrap. Gently pound until about 1/8 inch thick.
Heat oil in a large, heavy skillet. Dredge medaillons lightly in flour, gently shaking off any excess flour. Cook turkey in hot oil over medium-high heat for 10 minutes or until cooked through with no pink remaining, turning once. Garnish with lime slices, if desired. Serve immediately. Makes 4 servings.