Combine first 8 ingredients in a medium saucepan; stir in 1/4 teaspoon salt. Bring to a boil. Cover, reduce heat to medium-low, and cook 45 minutes or until onions are almost tender, stirring occasionally. Uncover, increase heat to medium-high, and cook 7 minutes or until thick, stirring frequently.
Heat oil in a large cast-iron skillet over medium-high heat. Sprinkle pork evenly with remaining 1/4 teaspoon salt and pepper. Add pork to pan, and cook 1 minute. Turn pork over. Place pan in oven; bake at 500° for 12 minutes or until a thermometer registers 155° (slightly pink). Remove from oven; let stand 10 minutes. Slice pork crosswise into 1/2-inch-thick slices. Serve with sauce.