For the lentil filling:

1. Heat oil in a large, heavy bottomed pan over medium heat, and once hot add the mushrooms and sear for about 5 minutes. Add the onions and cook until softened. Stir in the garlic, herbs, and spices; season with salt and pepper, and cook for an additional minute. Pour in the balsamic vinegar, stir.
2. Add the vegetable stock, increase the heat to high and bring to a rolling boil. Add the lentils and reduce the heat to medium-low, then simmer for 20-25 minutes, or until the lentils are cooked. Taste and adjust seasoning as needed.
3. In a small bowl, whisk the cornstarch with water and stir it in to the filling, stirring for a few minutes constantly until the mixture thickens. Remove from the heat and set aside.

For the pumpkin and sweet potato mash:

1. Place pumpkin, potatoes and sweet potatoes into a medium pot with water and a pinch of salt. Bring to a boil over high heat, then reduce to medium and simmer until tender, about 20 minutes.
2. Drain and mash, using a potato masher. Add milk, olive oil, paprika, salt and pepper, and stir well. Adjust seasoning as needed. Set aside.
3. Preheat the oven to 400°F (200°C) and grease a shallow baking dish with oil.
4. Pour the mushroom filling into the prepared dish and top with the pumpkin and sweet potato mash, spreading into an even layer.
5. Bake for 15-20 minutes, until golden and the sauce has started to bubble up around the edges. Serve immediately, with a side of green salad.