Peel and cut the sweet potatoes into 2cm chunks, then peel and cut the onions into 2cm-thick wedges.
Place in a roasting tray in an even layer and sprinkle over the cumin seeds, ground coriander and a pinch of sea salt and black pepper. Drizzle with oil, then toss to coat. Pop in the oven for 40 to 45 minutes, or until cooked and golden.
Peel and finely slice the garlic, then finely slice the chilli. Pick the coriander leaves and set aside, then finely slice the stalks.
Place a large saucepan over a medium-low heat and pour in a lug of oil. Sauté the garlic, chilli and coriander stalks for a minute, until lightly golden.
Add the red lentils to the pan. Stir to coat, then pour in the hot stock and coconut milk.
Turn up the heat, gently bring it to the boil, then let it come to a simmer. Cook the lentils for 20 minutes, or until they are completely broken down.
When the veggies are ready, remove from the oven and carefully spoon into the pan. Add most of the coriander leaves, then blitz the soup with a stick blender until it’s creamy but still has a little texture.
Squeeze in some lemon juice to taste and adjust the seasoning if need be. Serve the hot soup topped with the remaining coriander leaves and toasted coconut shavings, if you like.