Start by making the sweet potatoes. Cook two sweet potatoes, halved and cut-side down, on a baking sheet in a 200°C (400°F) oven for about 30 minutes. While they're baking, prepare the lentil base.
1 batch mashed sweet potatoes
Heat the oil in a large heavy-bottom pot over medium. Once the pot is hot, add the mushrooms. Cook, stirring as infrequently as possible, until browned and any water has evaporated.
Stir in the onions and cook for another five minutes, stirring occasionally, until softened. Add the garlic, herbs, and spices, and cook for another minute.
1 medium onion, 2 cloves garlic, 1 sprig rosemary leaves, 1 teaspoon sea salt, ½ teaspoon black pepper, ¼ teaspoon smoked paprika
Add the red wine and stir to release anything stuck to the bottom of the pot, then pour in the vegetable broth. Cover the pot and bring to a low boil.
50 ml dry red wine*, 500 ml vegetable broth
Once the broth is boiling, stir in the lentils. Turn the heat down to simmer and cook, covered, for 15-20 minutes, or until the lentils are softened. Stir in the thyme.
150 grams brown lentils, 1 tablespoon thyme leaves
While the lentil mixture is still simmering, mix the water and starch together in a small bowl. Stir this mixture into the lentils, then simmer for a couple minutes more to thicken.
1 tablespoon water, 2 teaspoons corn starch
Transfer the lentil mixture to a large baking dish and top with the frozen peas. Mix up the baked sweet potatoes with milk, olive oil, and salt, then spread over the pie filling.
Bake at 200°C (400°F) for about 10 minutes, or until you see the filling bubbling up around the edges of the dish. Serve hot.
Notes
* Substitute 2 teaspoons balsamic vinegar OR 1 tablespoon tomato paste (cooked with the garlic) for wine if preferred, and add extra broth to make up the difference.For the sweet potatoes you'll need 2 medium sweet potatoes, 50 ml milk, 1 tablespoon olive oil, and salt to taste.