Preheat the oven to 200 °C if using pastry. Defrost 2 sheets flaky puff pastry, if using. Bring a steamer pot of hot tap water to a boil. Bring a jug of water to a boil. Spray a baking dish with oil spray.
Cook mash
Peel and dice 400 g potatoes into small chunks. Cook the potato in the steamer pot of boiling water for 12-15 minutes until tender. Drain the water, then transfer the potatoes to the pot. Add 1 tbsp olive oil, ¼ cup milk, and season with salt and pepper. Mash until smooth.
Cook sausages
Meanwhile, cook 6 sausages in a large frypan on medium-high heat, tossing regularly, for 8-10 minutes or until cooked through. Set aside to cool before slicing into 1cm thick rounds. Reserve the pan.
Prep ingredients
Meanwhile, peel and finely dice 1 carrot. Peel and mince 4 cloves garlic. Trim 1 leek, halve lengthways, wash and thinly slice. Peel, core and dice 2 apples into 1cm chunks. Crumble 2 cubes chicken stock into a heatproof jug, add 1 cup of boiling water and 2 tsp Dijon mustard and whisk to combine.
Start filling
Heat 1 tbsp oil in the reserved pan on medium-high heat. Add the carrot, garlic, leek,apple, 1 tbsp Italian herbs, and 1 cup frozen peas to the pan, season with salt and pepper, cover and cook for 8-10 minutes.
Finish filling
Sprinkle 2 tbsp flour over the vegetables and cook, stirring, for 2 minutes until combined. Gradually add the stock, stirring constantly to prevent lumps from forming, and bring to a boil. Stir most of the sausages into the pan, along with any juices, then evenly spoon into the prepared baking dish.
Bake pie
Spread the mash over the top of the pie filling using the back of a spoon. Poke the reserved sausage slices into the top, spray with oil spray and crimp the edges with a fork to seal. Bake in the oven for 15-20 minutes or until the pastry is cooked and golden (if using pastry).Note: If not using pastry, grill in the oven for 5 minutes, or until golden on top.
To serve
Divide the bangers and mash style pie between plates and serve with tomato sauce on the side.