Heat the butter in a pan. Add the onion and carrot and cook for 5-6 minutes.
Add in the finely diced mushrooms and cook for another 3-4 minutes.
Add the lentils, white pepper, and nutmeg and stir through for a couple of minutes.
Preheat the oven to 180°C (360°F).
To the pan, add the pinhead oats, Marmite, and veg stock. Cook for 15 minutes on a low heat, stirring regularly. The mix should become quite thick, but add a splash more water if it seems too thick.
After 15 minutes, turn off the heat and stir through the chopped sunflower seeds.
Spoon the mix into a greased loaf tin and bake in the oven for approx. 30 minutes. To make the outside extra crispy, turn up the heat slightly towards the end.
Once cooked, remove from the oven and allow to stand for five minutes. Serve with neeps, tatties, cabbage or kale, and vegan whisky cream sauce. If you have any leftovers, why not treat yourself to a vegan haggis toastie, haggis fritter, or haggis quesadilla?