In a large frying pan add the oil, onion slices and pork sausages, cook over medium-high heat, browning on all sides. Cook until no pink remains in the middle of the sausages.
In a microwave safe container or bowl place potatoes and microwave on high for 13 minutes, or until a fork poked in goes in easily.
Preheat oven to 400 and line a baking sheet with aluminum foil and spray with cooking spray.
Remove potatoes from microwave and allow to cool for 5 minutes, then cut them in half and lay them flat side down on the baking sheet.
Take a spoon and press into the center of the potato half to 'smash' it. Repeat until all potatoes are smashed. Sprinkle potatoes with salt and garlic salt, then take little pinch fulls of cheddar and place it in the well created by smashing the potatoes. Bake for 7-10 minutes, or until the cheese is melted. Remove from oven and allow to cool for a few minutes.
To serve place 1 sausage and some onions on a plate and then place two potato halves onto the plate. Sprinkle the potatoes with parsley and enjoy!