Combine first 4 ingredients in a large zip-top plastic bag; seal. Marinate in refrigerator 2 hours, turning occasionally.
Preheat oven to 425°.
Remove pork from bag, reserving marinade. Strain marinade through a sieve, reserving solids. Set solids aside; discard liquid. Sprinkle pork evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Heat oil in a large ovenproof skillet over medium-high heat. Add pork to pan; cook 3 minutes, turning to brown on all sides. Bake at 425° for 12 minutes or until a thermometer registers 155°. Place pork on a cutting board; cover loosely with foil. Let stand 5 minutes. Cut into 1/4-inch-thick slices.
Combine reserved solids, remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, and 2 tablespoons water in a saucepan over medium-high heat. Cook 12 minutes or until onion is browned, stirring occasionally. Serve with pork.