Rinse the lentils under cold running water for about three minutes.
Dry roast the seeds until they start to pop, change colour and become fragrant. Grind the seeds in a spice grinder or mortar and pestle and set aside.
In a large saucepan saute one tbsp of olive oil, onion, chilli and garlic for 2-3 minutes on a low heat.
Add the tomatoes, breaking them into smaller pieces with a wooden spoon. Add the stock and the lentils and bring to the boil, before turning the heat down and simmering for 2 hours (until the liquid is completely absorbed into the lentils and the lentils are cooked but still retain their shape and texture).
While the lentils are cooking, steam the sweet potato for 15-20 minutes until it is cooked. Roughly mash the sweet potato and add it to the cooked lentils and place the mixture in a bowl to cool.
Mould with your hands into patties approximately 6 cm in diameter and 2cm thick. Dust very lightly with flour and fry over a low to medium heat for 4 minutes each side using the remaining olive oil.
Serve over a slice of wholegrain bread, with slices of tomato, avocado and salad.
Delicious accompanied with tzatziki, baba ganoush or chilli sauce.