In a 12-inch cast-iron skillet, heat oil over medium heat. Add pork; cook until browned all over, 1 to 2 minutes per side. Remove pork from pan.
In a large bowl, stir together butter, garlic, half of ranch seasoning, salt, and ½ teaspoon pepper until well combined; stir in potatoes and onion until coated. Add vegetables to skillet.
Bake for 20 minutes, stirring vegetables after 10 minutes.
In same bowl, stir together cheese, remaining ranch seasoning, and remaining 1 teaspoon pepper.
Push potatoes to one side of skillet. Add pork to empty side of skillet; sprinkle with cheese mixture.
Bake until potatoes are tender and an instant-read thermometer inserted in center of pork registers 145°, 15 to 20 minutes. Let stand for 5 minutes before slicing. Garnish with chives, if desired.
Recipe by Southern Cast Iron at https://southerncastiron.com/parmesan-ranch-pork-potatoes/