In the cooker, heat oil and add onions, green onion, garlic, celery and carrots, stirring until the carrots are fork-tender (about 7 minutes). Season with salt and pepper. Once finished, set aside so they are cool to the touch.
In a bowl, combine the cooled vegetables, Worcestershire sauce, ¼ cup ketchup, ground beef, ground pork, mushrooms, breadcrumbs, egg, milk, and Parmesan cheese.
Shape into an oval and place in the cooker.
Brush remaining ½ cup of ketchup over meatloaf. Close and lock the lid of the cooker and set the regulator knob to STEAM. Set the timer for 5 hours on slow cook LOW.
Once the cooking time is done, carefully remove meatloaf from cooker and tent with foil for 10 minutes. Slice and serve.