Place lentils and chickpeas in a large sieve. Rinse under cold water. Press with the back of a spoon to extract excess liquid.
Process chickpeas, lentils and garlic in a food processor to form a coarse paste. Transfer mixture to a large bowl. Add coriander, tahini, flour and harissa. Mix well.
Divide evenly into 6 portions. Shape each portion into an 8cm round patty.
Spread breadcrumbs over a large plate. Coat patties in breadcrumbs, pressing on lightly. Transfer to a tray.
Heat oil in a large, non-stick frying pan over a medium-high heat. Add patties. Cook for 4-5 minutes on each side, or until golden brown.
To make chilli yoghurt, combine all ingredients in a small bowl.
Top base of rolls with lettuce, tomato, cucumber and hot patties. Spoon chilli yoghurt over patties. Replace roll tops to serve.

Harissa is a North African spice and herb blend. It is available from the spice section in supermarkets and fruit and vegetable stores.
For a lighter meal, omit rolls and serve patties with a leafy salad.
Omit chilli from yoghurt for a milder topping.
Patties can be prepared and cooked up to 2 days ahead. Keep, covered in fridge.
Reheat in oven or microwave.