Rehydrate mushrooms if using dried (~20 mins).
Peel and slice potatoes, add to a large pot of salted water and boil on medium-high heat for ~15 mins until soft.
For the filling, dice the carrots (1cm/½ inch cubes), and add to another large pot. Coat with the garlic and onion oil and start to saute on medium heat.
Meanwhile, finely chop the mushrooms in food processor and add to the carrots along with the thyme, sage, paprika, salt and pepper and continue to saute for a few mins.
Stir in the tomato paste and flour to coat the mushrooms and carrots. Slowly add the stock/broth, wine, and optional marmite/soy sauce, stirring continuously. Let the filling simmer for a few minutes to reduce and thicken.
Pre-heat the oven to 200°C/400°F. Drain and thoroughly rinse the lentils and add them to the filling to heat through.
Drain and mash the potatoes with butter until smooth and fluffy, season to taste.
Tip the filling into the baking dish (a deeper dish is better than a shallow one). Top with the mashed potato. Create ridges with a fork and top with cheese if using.Tip: To avoid the mash sinking into the warm filling, take a baking tray/sheet and line with baking paper/parchment. Smooth the potatoes into an even layer on the tray to roughly the size of the baking dish. Line up the tray to the dish, holding the tray and paper, swiftly flip the potato topping onto the filling and neaten up the edges. I came up with the flipping method when I was in a rush and it worked amazingly well. But if you have a bit of time, wait for it to cool and thicken a bit before adding the mash and heat for longer in the oven. Put the pie in the oven for ~20 mins to heat through (~45 mins or until piping hot if cooking a pre-prepared pie from cold). Grill/broil the pie for the final ~5 mins to get a crispy top (keep a close eye to catch it from burning).