For the filling, soak lentils in a large bowl covered in plenty of cold water overnight (see note). Drain and rinse well under cold running water. Transfer to a large saucepan, cover with cold water. Place over medium-high heat and bring to a simmer, stirring occasionally. Simmer, stirring occasionally, for 18-20 minutes or until just tender. Drain well. Set aside to cool to room temperature.

Heat half the oil in a large frypan over medium-high heat. Cook the onion, celery, garlic and kale for 3 minutes, stirring occasionally, until softened. Transfer to a food processor. Heat the remaining oil in same pan over high heat. Cook mushroom, stirring occasionally, for 5 minutes or until golden and all moisture has evaporated.

Add to food processor. Season and whiz until a coarse mixture. Set half aside in a large bowl. Whiz remaining half with pumpkin seeds, soy sauce and tomato paste until smooth, scraping down side of bowl occasionally, then transfer to coarse mixture in bowl. Add all the remaining ingredients, except for extra small whole herb leaves, and the lentils, stir until well combined and season to taste. Preheat oven to 230°C/210°C fan-forced.

Brush a 24cm x 13cm, 6.5cm-deep loaf pan with extra oil, then sprinkle base of pan with extra small whole herb leaves. Place a baking tray in oven on a lower shelf to catch any juices that spill over.

Place maple syrup, cinnamon and thyme in a large bowl and season with salt flakes and freshly ground black pepper. Using a mandoline, cut sweet potato lengthwise into 2.5mm-thick slices directly over the bowl, then toss until well coated in maple syrup mixture. Line base and sides of prepared pan with overlapping slices of sweet potato, hanging slices over sides, as they will be used to enclose the filling.

Pack filling tightly into sweet potato-lined pan and fold over the overhanging sweet potato to cover the filling. Cover with a sheet of baking paper and then with a small baking tray weighted down with a heavy baking dish. Bake for 25 minutes. Remove baking weight, tray and baking paper from top of the loaf and bake for 25 minutes or until top is deep golden and shiny. Remove pan from oven and stand on a wire rack for 5 minutes to allow the loaf to rest.

To serve, turn loaf out onto a serving plate and brush with any maple syrup that remains in the bottom of the pan. Serve sliced with salad and mustard fruit.