Preheat the oven to 200°C fan.
Put the diced potato into a medium saucepan of cold water, cover and bring to the boil. Take care not to let the water boil over. Lower the heat, partially cover and simmer for about 15 minutes until just cooked through, taking care not to overcook the potatoes. Remove from the heat, drain well and set aside.
Meanwhile, spray a medium frying pan with low-calorie cooking spray and place over a medium heat. Add the onion and leek and cook for 5 minutes or until starting to soften.
Add the sausages, 1 tsp of the garlic granules, mustard powder and sage. Cook for 10-15 minutes over a medium heat until the sausages are sealed and lightly browned on all sides, stirring occasionally.
Pour in the stock and cook for 2 minutes. Stir in the spreadable cheese, then remove from the heat. Make a well in the centre of the mixture and add the quark. To prevent the quark from splitting, work from the centre out, stirring until fully combined. Stir in the frozen peas and season to taste with black pepper.
Transfer the mixture into a 24cm diameter round ovenproof dish. Sprinkle half the Cheddar cheese evenly over the top, then cover with the cooked potato.
Sprinkle the remaining garlic granules over the top and place, uncovered, in the oven for 15 minutes.
Remove from the oven, sprinkle the remaining cheese over the top and return to the oven, uncovered, for a further 15 minutes until bubbling and golden on top. Serve hot, alone or with your choice of accompaniment.