Add diced onion to a large saucepan and fry in a little oil until translucent. Add lentils and stock and cook until lentils are mostly soft. Add frozen veggies and cook until all the water is absorbed and the vegetables are soft. Mix through thyme and season with extra salt and pepper to taste. Press lentil mixture down evenly into the base of small oven safe dishes or ramekins.
Chop potatoes into medium-sized cubes and place in a saucepan. Cover potatoes with water, season with salt and pepper to taste. Boil until potatoes are soft and easily pierced with a fork. Drain excess water and mash potatoes with a potato masher. Add vegan butter if using and mix through. Mix in soy milk until potatoes are creamy. Spoon mashed potatoes over the lentil mixture and cover evenly.
Place dishes to bake in the oven at 200 C for about 20-30 minutes or until the potatoes start to form a brownish crust on top.
Remove from oven and garnish with chopped parsley. Serve hot.