Heat oil and butter in cooker. Add pork and brown evenly on all sides. Remove pork. Stir-fry onion and carrot in remaining oil for about 2 minutes. Remove from heat. Place trivet or rack on bottom of cooker. Add stock and water. Return pork and vegetables to cooker. Sprinkle pork with salt and pepper. Return to heat. Lock lid and bring to full pressure. Lower heat, maintaining pressure, and cook for 37 minutes. Remove cooker from heat and let pressure drop naturally. Remove pork to serving platter. Remove trivet, discarding carrots. Return juices in cooker to a boil and cook until liquid is reduced by half, stirring occasionally. Serve sauce over hot pork.