Preheat the oven to 200°c.
Spray a large saucepan with low calorie cooking spray and add the onions. Gently fry them over a medium heat for 5 minutes until they have softened slightly.
Add the beef mince to the saucepan and cook for 5 minutes until browned.
Add the swede, carrot and potato to the pan and sweat for 5 minutes.
Add the stock, Worcestershire sauce, Marmite, thyme and garlic granules and stir to combine. Put the lid on the saucepan and simmer for 15 minutes, stirring occasionally. When done, the vegetables will have softened enough to easily pierce with a fork but shouldn't have turned to mush.
Take the lid off the pan. The mix should be nice and thick, if it still seems a little watery then leave the lid off and reduce any remaining stock. Season with salt and pepper to taste.
When the mince mixture is ready, transfer it to an oven proof dish ready to add the pastry.
Slice the sheet of pastry into strips about the thickness of a pencil. Lay the strips across the top of the pie in one direction leaving gaps between the pieces. Then add strips in the opposite direction to create a grid pattern.
Spray the top of the pie with low calorie cooking spray and place into the oven for 10-15 minutes (depending on the instructions on your pastry) until the pastry has puffed up and turned golden. Slice and serve with a side of your choice.