Make the mashed potatoes first. Boil the peeled and cubed potatoes until soft, about 15-20 minutes. Drain, add the potatoes back to the pot, and add the milk, sour cream, butter, garlic salt, and pepper, to taste. Mash the potatoes with a potato masher or fork until you achieve your desired texture. Add a little bit more milk, if needed, to make them smooth if you don’t like lumps. Set the potatoes aside.
Preheat the oven to 190°c. Heat a large pan over medium heat and add the oil, mushrooms, onion, and garlic to the pan. Fry until the onion turns translucent, about 5-10 minutes, stirring occasionally. Add the lentils, mixed vegetables, soy sauce, tomato paste, vegetable stock, dried parsley, salt, and a pinch of pepper. Stir to combine the mixture. Heat for another 5-7 minutes, stirring occasionally.
Spoon the mixture into a pie dish or an 8×8-inch square baking dish and spread it into an even layer. Carefully dollop and spread 630g of the mashed potatoes over top of the vegetable mixture and bake for 20 minutes. (We like to place pie/baking dish on a baking tray in case it bubbles over.) Turn the oven to grill and grill for another 5 minutes, until the mashed potatoes start to brown on top. Let cool for about 5-10 minutes before serving.
Note: You will have leftover mashed potatoes to enjoy with another meal during the week or serve to guests separately as a side.
