While the potatoes are boiling, heat 3 tbsp of olive oil in a large pot over low/medium heat.
Finely chop 1 onion and 2 carrots, and add them to the pan. Cook for 5-7 minutes until softened.
Add 300g of chopped mushrooms and continue to sauté for another 5 minutes until the mushrooms reduce in size and release their moisture. Increase the heat to get rid of all the water released from the mushrooms, it needs to evaporate.
Stir in 1 tbsp of soy sauce or tamari, and 2 tbsp of tomato paste and cook for 1 minute. Then add in 2 cans of lentils drained (about 500g drained eeight), and mix well to ensure all ingredients are coated evenly. Let it cook for 2-3 minutes.
Sprinkle in 1/4 tsp of paprika, 1/4 tsp of dry parsley or other dry herbs, and 1 tbsp of plain flour or oat flour. Mix to combine and cook for another 2 minutes. Finally season with salt and pepper.
Transfer the filling into an oven-safe baking dish, spreading it evenly.
Spoon the mashed sweet potatoes over the filling, spreading it carefully to cover the entire surface. Use a fork to create ridges on top, which will crisp up during baking.
Place the pie in the preheated oven and bake for 40 minutes at 180°C (350°F).
For a golden finish, switch the oven to grill (broil) for the last 5 minutes until the top becomes slightly crisp.