Peel and finely chop the shallots. Heat a large pan. Tip in 1 tsp cumin seeds and toast for 2-3 mins till popping and spicy smelling. Transfer them to a small bowl. Keep the pan on the heat and drizzle in 1 tbsp olive oil. Add in the shallots, season, and cook for 5 mins on a low heat, till softened.
Trim and peel the carrots. Coarsely grate them. Crush 5 cardamom pods. Discard the papery skins and save the black seeds.
Add the cumin seeds and cardamom seeds to the pan with 1 tsp turmeric. Stir together for 1 min. Add the carrots. Stir to coat in the spiced shallots.
Tip in the red lentils. Crumble in the stock cube and pour in 700ml boiling water. Pop a lid on the pan and simmer for 20 mins.
While the soup simmers, trim and finely slice the salad onions, both the white and green bits.
Squeeze in half the lime. Taste and season the soup, adding more lime juice if you think it needs it. Blitz the soup till smooth using a stick blender or transfer to a blender to blitz. Top with the slivers of salad onion to serve.