Preheat the oven to 200C.
Peel & chop up the sweet & white potatoes into chunks. Fill a pot with water & wait for it to boil, add the chopped potatoes.
Dice celery, garlic & onions.
Drizzle olive oil into a pan, add diced onions, garlic & let them soften & sweat. Then add the diced celery until they begin to soften.
Add lentils, tomatoes & into the pan stir until thoroughly mixed. Add tomato paste, oregano, stock cube, salt & pepper.
Check the potatoes - poke the chunks with a fork. If the fork goes through. Turn off the heat & drain the potatoes. Mash up the softened potatoes with some butter, salt & pepper.
In a baking tray, add the cooked lentil mix, followed by the mashed potato. Spread the mashed potato with a fork. Add the shredded cheese until it covers the top.
Bake pie until the top turns golden brown, approx. 15 minutes.