Place the potatoes in a large pot and cover entirely with water. Bring to a gentle boil and then simmer for around 15 minutes or until you can cut through them easily with a knife.
While the potatoes are cooking, heat the oil in a large pan on medium heat. Add the chopped onion and carrot and soften for 6-8 minutes, stirring occasionally.
Next, stir in the chopped garlic and cook for another 2 minutes before adding the chopped mushrooms (both the larger chunks and finer pieces). Stir well and cook for 5-6 minutes, turning up the heat if necessary.
After 5-6 minutes, stir in the tomato puree, green lentils, Marmite, Henderson's Relish, vegan stock, bay leaves, dried herbs, salt, and pepper. Mix well, reduce the heat, and simmer for 10-15 minutes.
Preheat the oven to 200°C (390°F).
Once the potatoes are cooked, drain them and return to the pot. Add the plant-based butter, unsweetened plant-based milk, salt, and pepper, as well as the nutritional yeast and/or wholegrain mustard if including these. Mash the potatoes with a masher until they are smooth and fluffy, adding more of any ingredient you like.
After simmering the sauce uncovered for 10-15 minutes, it should mean that there's no need for any thickening agent. If it looks too thin, you can stir a spoonful of the mash through the sauce to thicken it. (You want the filling to be quite thick so that the mashed potatoes sit on top of it without sinking).
Adjust the seasoning if necessary, before transferring the filling to a large oven dish and spreading evenly. Carefully spoon the mashed potatoes on top, starting at the corners and edges and working your way in. Smooth out with a spoon or spatula and then make small marks all across the surface using a fork.
Add some small dots of plant-based butter on top of the mash and then bake in the oven for 30 minutes. To get it even crispier, you can switch to the grill for the last 5-10 minutes.
The cottage pie should be golden on top, with some of the sauce making its way to the surface. Serve with chopped fresh parsley and greens of your choice.