Heat a small saucepan over medium heat. Coat pan with cooking spray. Add onion, ginger, and garlic; sauté 2 minutes. Add water and sugar; bring to a boil. Cook until reduced to 1/2 cup (about 5 minutes). Remove from heat; carefully stir in soy sauce, vinegar, and mustard. Add butter, stirring with a whisk. Set aside; keep warm.
Preheat oven to 350°.
Rub tenderloins evenly with crushed peppercorns, thyme, and salt. Heat a large ovenproof nonstick skillet over medium-high heat. Coat pan with cooking spray. Add tenderloins, browning on all sides (about 5 minutes). Bake at 350° for 23 minutes or until a thermometer registers 160° (slightly pink); let stand 10 minutes. Cut each tenderloin into 12 slices; serve with sauce.