1. Cook pancetta in medium skillet over high heat until crisp, about 2 minutes.
2. Bring cream, onion, and tarragon sprigs to boil in heavy small saucepan. Remove from heat. Cover and steep 25 minutes. Strain mixture into saucepan. Boil tarragon cream until reduced to 1/4 cup, about 4 minutes. Whisk in chopped tarragon and mustard. Season with salt and pepper. Keep warm.
3. Cut each tenderloin crosswise into 4 pieces. Using mallet, pound each piece between sheets of plastic to 1/4-inch thickness. Sprinkle with salt and pepper. Heat oil in large skillet over medium heat. Add turkey and sauté until cooked through, about 2 minutes per side. Transfer turkey to 4 plates. Spoon tarragon cream over and sprinkle with pancetta.