In a large saucepan on high heat, boil potatoes until just tender, but not falling apart. Remove from pan and allow to cool. Gently slice and set aside.
Meanwhile, heat oil in a large ovenproof frying pan on medium-high heat. Add sausages and cook, turning as needed, until browned and almost cooked through. Remove from pan. Add onion, tomato and garlic and cook until onions are tender. Remove from pan and set aside.
Stir together gravy powder and water in a jug until thoroughly combined and pour into pan. Allow to simmer gently and cook, stirring regularly, until thickened; about 5 mins.
Add sausages and onions back to the pan and gently layer over sliced potatoes. Season with garlic salt and top with cheese. Bake for 35-40 mins or until cheese is golden.