Cook Lentils: Rinse 500g green lentils well and cook according to packet instructions, about 25 minutes on medium-low heat until soft but not mushy.
Prepare Sweet Potatoes: Chop purple and white sweet potatoes with skin on and steam for about 25 minutes until soft.
Cook Vegetables: In a pot, cook chopped mushrooms without oil or with 1 tbsp olive oil until softened and moisture released. Add chopped carrots, celery, and onion, with a small splash of water to prevent sticking. Season with mixed herbs, tarragon, crushed garlic, black pepper, and salt to taste. Cook gently on medium heat until vegetables are soft and aromatic.
Combine Lentils and Tomatoes: Add drained cooked lentils, a tin of chopped tomatoes, and cornflour slurry (2 tbsp cornflour mixed with water) to the pot. Pour in about 200 ml water, mix well, cover, and simmer for about 10 minutes.
Prepare Sweet Potato Topping: Lightly crush the steamed sweet potatoes leaving some chunks for texture. Optionally, dress with olive oil, vegan butter, salt, pepper, or plant milk, or keep simple.
Assemble and Bake: Spoon the lentil mixture into a large baking dish and spread the crushed sweet potatoes evenly on top. Bake in a preheated oven at 200°C for about 20 minutes until the top is golden and slightly crisp.