1. Cut pork tenderloin into 4 pieces; use meat mallet to pound each out to 1/2-inch thickness. Set aside.
2. For marinade, whisk together lemon juice, orange juice, vinegar, 1 tablespoon oil, soy sauce, adobo seasoning, oregano and salt in large bowl. Stir in garlic, onions, chile and shallot if using. Add pork tenderloin cutlets to marinade making sure they are well coated. Let marinate 30 minutes at room temperature. Remove pork from marinade and place on sheet pan; reserve marinade.
3. Strain marinade mixture. Heat 1 tablespoon oil in large skillet. Add strained onion mixture to skillet; cook and stir over medium-high heat until onions are tender and have caramelized, about 5 minutes. Remove from skillet.
4. Heat remaining 1 tablespoon oil in skillet; add pork cutlets and cook until internal temperature reaches 145°F to 160°F, about 4 minutes per side. Remove from skillet; let stand 3 minutes.
5. Add reserved marinade to skillet; bring to a boil over medium-high heat; cook and stir until slightly thickened. Serve pork cutlets topped with caramelized onions and drizzled with thickened marinade.