Put the peppers and onion on a baking sheet. Drizzle over the oil, season with salt and pepper and toss until evenly coated in the oil, then spread out in a single layer. Roast in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes, turning once and adding the garlic cloves halfway through, until the peppers are starting to char and the onions are soft.
Squeeze the soft garlic out of its skin and place in a saucepan. Add the roasted peppers to the pan with the onion and any oil from the baking sheet. Add the lentils, chilli flakes, cumin and stock and bring to the boil, then cover and simmer for 20 minutes until the lentils are soft.
Transfer the soup in batches to a blender or food processor and blend until smooth. Return to the pan and reheat through. Ladle into bowls, sprinkle over the toasted seeds and garnish with chopped coriander.