1. Preheat oven to 450°.
2. To prepare sauce, combine first 4 ingredients in a saucepan; bring to a boil. Cook until reduced to 1/2 cup (about 18 minutes). Remove from heat. Gradually add butter; stir with a whisk until blended. Stir in vinegar and 1/4 teaspoon salt.
3. To prepare pork, while sauce reduces, combine fennel seeds, garlic powder, cumin, 1/4 teaspoon salt, and pepper; rub over pork. Heat oil in a large ovenproof skillet over medium-high heat. Add pork; cook 2 minutes on each side or until browned. Place skillet in oven. Bake at 450° for 18 minutes or until a thermometer registers 155° (slightly pink). Let stand 5 minutes; cut into 1/4-inch slices. Serve with sauce.