Place the potatoes, carrots and swede in a pan and cover with cold, salted water. Bring to boiling and cook for around 20 minutes until soft.
While the mash topping cooks, spray a large saucepan or sauté pan with low-calorie cooking spray and place over a medium heat.
Add the leeks, onions and carrots and cook for 5 minutes until soft. Add the garlic and thyme and cook for another minute.
Add the mushrooms and cook for a further 5 minutes.
Add the tomato puree, stock, stock pot (if using) and Henderson's relish. Stir in the nuts.
Bring to a simmer and cook for 10-15 minutes, stirring occasionally, until you have a thick consistency.
Preheat the oven to 200°C.
Drain and mash the potato, carrots and swede. Taste and season with salt and pepper, if required.
Pour the filling into an ovenproof dish. Top with the mash and fluff up the top with a fork.
Bake in the oven for 20-25 minutes, until the top is golden.
Serve!